Before the Altar of the world in flower,

Upon whose steps thy creatures kneel in line,

We do beseech Thee in this wild Spring hour,

Grant us, O Lord, thy wine. But not this wine.

Helpless, we, praying by Thy shimmering seas,

Beside Thy fields, whence all the world is fed,

Thy little children clinging about Thy knees,

Cry: 'Grant us, Lord, Thy bread!' But not this bread.

This wine of awful sacrifice outpoured;

This bread of life - of human lives. The Press

Is overflowing, the Wine-Press of the Lord!

Yet doth he tread the foamings no less.

These stricken lands! The green time of the year

Has found them wasted by a purple flood,

Sodden and wasted everywhere, everywhere;

Not all our tears may cleanse them from that blood.

We knew the sword accursed, yet with the strong

Proclaimed the sword triumphant. Yea this wrong

Unto our children, unto those unborn

We did, blaspheming God. We feared the scorn

Of men; men worshipping pride, so where they led

We followed. Dare we now lament our dead?

Shadows and echoes, harlots! We betrayed

Our sons; because men laughed we were afraid.

That silent wisdom which was ours to kept

Deep buried; thousands perished; still we slept.

Children were slaughtered, women raped, the weak

Down-trodden. Very quiet was our sleep.

There was no sound at all, no crying in the village,

Nothing you would count as sound, that is, after the shells;

Only behind a wall the low sobbing of women,

The creaking of a door, a lost dog-nothing else.

Silence which might be felt, no pity in the silence,

Horrible, soft like blood, down all the blood-stained ways;

In the middle of the street two corpses lie unburied,

And a bayoneted woman stares in the market-place.

Humble and ruined folk-for these no pride of conquest,

Their only prayer: "O Lord, give us our daily bread!"

Not by the battle fires, the shrapnel are we haunted;

Who shall deliver us from the memory of these dead?

Theirs was a great romance but one doomed to fail: the handsome, illegitimate ploughman's son and the earl's daughter, the beautiful Catholic socialite poet and Britain's first Labour Prime Minister, a low-Church Scot.

For the first time today, The Daily Telegraph reveals the love letters that James Ramsay MacDonald, the pre-war political giant who three times led the country, sent to Lady Margaret Sackville, youngest child of the 7th Earl de la Warr, during a 15-year relationship that was an absolute secret to all but a few of their closest friends.

The letters, of which there are about 150, show that MacDonald proposed at least three times to Lady Margaret, who was 15 years his junior. She turned him down each time. But he continued to shower her with intimate letters, writing poems and limericks and imagining a secret world in which they would be able to live without public comment.

The first of the letters, newly discovered at the National Archives in Kew, was written in 1913, a year after MacDonald met Lady Margaret, a famous beauty.

His biographer, David Marquand, the former President of Mansfield College, Oxford, said: "These letters are quite extraordinary and show a side of MacDonald that we have never really seen before."

Ramsay MacDonald, born in Lossiemouth in 1866, had just lost his wife, also called Margaret, to blood poisoning in 1911.

Lady Margaret Sackville, a poet who mixed with writers such as W B Yeats and Wilfred Scawen Blunt, was a friend of Lady Ottoline Morrell, a leading member of the Bloomsbury Set, and it is possible the lovers met through her.

Lady Ottoline, in her memoirs, had described MacDonald and Lady Margaret as "good friends", but the tone and hidden language of the letters shows that theirs was a full-fledged love affair.

MacDonald refers to sex in a repeated euphemism in his letters. Occasionally, he describes his marriage proposals as a "pilgrimage".

In a letter dated September 1915, just after he had proposed for a second time, MacDonald wrote: "Our love is wholehearted and cannot be changed and it is the most natural thing in the world that we should enter its holiest place."

But he then went on to indicate why Lady Margaret, who died unmarried in 1963, had refused to be his wife: "Nothing but the most formal barriers keep us from marrying and being with each other in a full common life and being all the better and happier in consequence.

"But, as you say, there are friends who, if they knew, would not understand or sympathise and of course they might come to know.

"You know, my dearie, how much I should grieve if your love for me brought you into conflict with anyone."

As well as worries about what her aristocratic family and friends might think, there was also the question of religion. Brought up in the Presbyterian church, MacDonald later joined the even lower Free Church of Scotland: Lady Margaret was Roman Catholic. David Marquand speculated that this was what the Labour pioneer meant by "the most formal barriers".

But another letter almost certainly written in the following year suggests that the idea of marriage had been reconsidered.

In it, he imagines that a friend has written to the local newspaper in his Midlands constituency, announcing the banns: "Sir, the honour of Leicester is to be enormously enhanced. Your [MP] — a poor misguided creature at present — is about to ally his fortunes to a famous house celebrated for its age, the beauty of its women, and the Conservatism of its men."

On occasions he was writing to Lady Margaret twice a day, sometimes arranging to meet, sometimes discussing the politics of the anti-war movement they both belonged to and which made them despised in the patriotic Britain of the Great War. But most of the letters are simple expressions of love for a woman with whom he spent too little time.

One undated letter reads: "Dearest beloved, it is such a beautiful morning that you ought to be here and we should be walking in the garden.

"And if we were walking in the garden, what more should we do where the bushes hid us?"

Another letter contains two limericks, one of which refers to the beautiful aristocrat's fondness for taking the waters and the strange health offerings of the Spa Hotel in "Strath", or Strathpeffer, in the Highlands of Ross-shire:

"A Lady went up to the Strath

For radium drinks and a bath

Her sweetheart turned up

And she flung down her cup

And kissed him to death in her wrath."

The collection of letters, which were kept in the home of Lady Margaret's bank manager in Cheltenham after her death in 1963 before being handed to the Historic Manuscript Collection, is incomplete. But those that still exist clearly show how the relationship waxed and waned, with the letters charting Lady Margaret's occasional desire to keep MacDonald at a more discreet length.

Immediately after the war, when he suffered for his pacifism with a heavy defeat in the 1918 election, MacDonald was in the wilderness and spent some time in Vienna, where he was rumoured to have had an entanglement with a well-known courtesan.

But he seems to have renewed his relationship with Lady Margaret sufficiently that, in 1924, during his first, nine-month stint as Prime Minister, MacDonald invited her to stay the night with him at Chequers, the premier's official residence in Buckinghamshire.

Later, after the affair petered out, MacDonald fell in love with Lady Londonderry, the wife of one of the ministers in his National Government, but nothing in his life ever again touched the depths of his feelings for "my own dearie".

It was a passion they could not make public, a love doomed to be declared in scribbled letters or stolen moments when they walked together. Ramsay MacDonald was the ambitious, illegitimate son of a farm labourer who became the first Labour prime minister. Lady Margaret Sackville was the youngest child of the seventh Earl de la Warr, a poet and a society beauty who became his lover.

They were separated not only by class but by religion. Born in Lossiemouth, Morayshire, MacDonald was raised in the Presbyterian church and, as an adult, joined the Free Church of Scotland. Born in Mayfair, London, and nearly 15 years his junior, Lady Margaret was Roman Catholic. But they met shortly before the first world war and found a shared commitment to pacifism and love of poetry.

For 15 years they were bound together in an intense relationship expressed in hundreds of ardent love letters written in black ink by MacDonald, which were kept by Lady Margaret but only rediscovered in the National Archives at Kew this week. They reveal a love that burned fiercely but could never be sealed in marriage.

MacDonald was nursing a broken heart when they first met. His wife, also called Margaret, had died from blood poisoning in 1911, the year that MacDonald became leader of the Labour party. It is possible that MacDonald, a widower with six children, was introduced to Lady Margaret by Lady Ottoline Morrell, a leading member of the Bloomsbury set who politely described the pair as "good friends" in her memoirs.

By the time of the first surviving letter, dated 1913, MacDonald, then 46, was already addressing Lady Margaret as "my dear heart". Two years later, the full horror of the war was unfolding and MacDonald had already experienced the first setback of his turbulent political career, forced to resign as party leader for his opposition to British involvement in the conflict.

As he swept from pacifist meeting to political rally, he diligently wrote to Lady Margaret, "my own dearest" and "my dear one". At times he would post two letters a day. MacDonald was known in parliament for his occasionally woolly rhetoric, but in private he was more direct, seldom shying from speaking of physical desire but couching it in a fantasy world of "the forest".

"My dear one," he wrote in June 1915. "That was a very loving letter I had from you yesterday. I feel its kisses. It brought you with it and I slept with my head on your breast last night after we have been in the very thickest places of the jungle together." Similar entries and letters continued throughout the summer. "Do you dream that I come to you?" he wrote. "Do I come to you when you are not dreaming? Do I kiss you and lie on your breast? Give me all the news about yourself and your heart and tell me all about your love."

A glamorous figure with a fondness for fur-lined jackets, Lady Margaret returned his passion with letters of her own. MacDonald was meticulous in conveying details of hotels where he was staying so that she could write. One day in 1915, he thanked her for some flowers. They were, he wrote, "fragile like kisses". On other occasions, it seems she gave toys to his children.

As MacDonald piloted his way through a political career that would see him become the prime minister of three governments, he had less fortune in persuading his lover to abandon propriety and marry him. From his letters it appears he asked for her hand in marriage three times and was rebuffed on all occasions. "It was so refreshing to see you again and so hard to part with you," he wrote in the spring of 1915. "I am sure it is right that we should not marry but what heartaches you give me! You are my own loved one and I want you always."

They shared their own secret world. MacDonald created playful fantasies that spoke of how he missed her. A keen photographer who kept numerous albums, he wrote of the photographs of her hanging on his walls. "Your photographs are misbehaving again very badly," he said in July 1915. "One in gorgeous evening dress in a hoity-toity way says: 'You cannot take me into the jungle, poor dear, because my dress would get crushed, so I wink at you maliciously and challenge you to embrace me.' Another says: 'Poor dear, you cannot speak to my heart because you cannot unloosen my brooch.'"

Lady Margaret was a protegee of the poet Wilfrid Scawen Blunt and known for her anti-war poems. Unintimidated despite never having gone to university, MacDonald would quote Burns in his letters or offer jaunty doggerel of his own. When Lady Margaret stayed at the Spa Hotel in Strathpeffer in the Highlands, he gave her "kisses, warm and invigorating as mud baths, fresh as the morning, and as full of love as your own heart", and a limerick:

"A Lady went up to the Strath

For radium drinks and a bath

Her sweetheart turned up

And she flung down her cup

And kissed him to death in her wrath."

Amid the playfulness, MacDonald was acutely aware of what the public, and the media, would say - whether it was the resentment of the recently enfranchised working class for his affair with an anti-war aristocrat, or the disapproval of the ruling class. He tried to make light of it, despatching one missive consisting of imaginary extracts from the London Mail and John Bull (which that year suggested MacDonald had deceived the public by trying to conceal his illegitimacy by using a false name).

"We congratulate Mr Ramsay MacDonald. The companion with whom he walked in Surrey last week would have done honour to kings and added grace to counts," he wrote. Then he poked fun at Margaret's self-consciousness at their stepping out in public and refusing to walk on his side of the path. "When we next meet them we hope to notice an improvement in this matter," he joked in the mock London Mail editorial.

Over time, MacDonald became more sensitive to the moments when his declarations were not returned with the same fervour. At times, he was plaintive: "The post has come [ ... ] Not a line, not a kiss, not a whiff of fragrance." Or questioning: "Why your silence? Have you eaten chocolates until you ache?" He could also be playful. Instead of "my own dear", he would begin letters, "my dear provocation" and "my dear deserter" and wrote: "Not a line! Not a kiss! Not a smile! Not a compensating frown!"

In the political wilderness after the war, MacDonald travelled to Vienna, when it was rumoured he became romantically involved with a similarly aristocratic Austrian socialite. But he continued to send love letters to Lady Margaret, speaking of "something ethereal like kisses" in 1923. When he first became prime minister in 1924 he wrote to her on 10 Downing Street embossed notepaper with the envelope - again kept by Lady Margaret - stamped "the prime minister". Making arrangements for her to stay at Chequers, the prime minister's official residence in Buckinghamshire, he wrote: "So I shall expect to see you on Saturday to stay that night" and, instead of the customary "ever, R", signed off with five kisses.

A decade on from the height of their passion, he was still moved to an elaborate metaphor in a 1925 letter, comparing a posted kiss to something as exotic and - in those days - difficult to transport as "an imported mango" from "a thousand miles away". However, he concluded, "there is nothing better to be had. So here is one carefully selected."

Later that year he wrote, again playfully, that she owed him a letter. "Perhaps you are dead; perhaps you are playing chess; perhaps you have fallen in love; but whatever has happened to you, I had better be wary and not intrude without sending in my card."

Why did their love wither away? Was it because they felt they could never marry without scandal and, perhaps, the sacrifice of MacDonald's political career? David Marquand, the former president of Mansfield College, Oxford, and MacDonald's biographer, has said he thought "the formal barriers" that the politician wrote were keeping them "from marriage ... and being all the better and happier in consequence" were the question of religion.

By the time of the final surviving letter, in 1929, the hectic meetings and conferences of a politician's life appeared to be getting in the way. "My dear, I have been trying hard to get a moment to write but for days engagements have fitted into the hours like pieces of a Chinese puzzle. What a life!"

For her part, Lady Margaret stayed true to her strong feelings for the iconic Labour leader who she knew as a passionate, playful lover. For nearly three decades after his death, and until her own in 1963, Lady Margaret kept his letters secret - and safe.