It (the Zinoviev Letter) took about a week to reach London and, having been evaluated by Desmond Morton, was circulated by SIS on 9 October to the Foreign Office and other departments. A covering note said that the document contained "strong incitement to armed revolution" and "evidence of intention to contaminate the Armed Forces", and was "a flagrant violation" of "the Anglo-Russian Treaty signed on the 8th August". Though, apparently, no systematic checks had been made, SIS also categorically vouched that "the authenticity of the document is undoubted".

The Foreign Office, nevertheless, carefully sought further corroboration from SIS. This was provided by Desmond Morton on 11 October based (he maintained) on information received from "Jim Finney" (code-named "Furniture Dealer"), one of the agents jointly run with Makgill's organisation, who had been infiltrated into the Communist Party of Great Britain. According to Morton, Finney reported that the Party Central Committee had recently received a letter of instruction from Moscow concerning "action which the C.P.G.B. was to take with regard to making the proletariat force Parliament to ratify the Anglo-Soviet Treaty" and that "particular efforts were to be made to permeate the Armed Forces of the Crown with Communist agents". This, concluded Morton, "seems undoubtedly confirmation of the receipt by the C.P.G.B. of Zinoviev's letter". But the original report contained no reference to any particular communication from Moscow, and Morton said he had ascertained details of a specific letter only during a subsequent meeting with the agent. Reflecting how curious it was that the agent had not mentioned so apparently significant a directive from Moscow in the original report, Milicent Bagot, a retired

M15 officer who spent three years in the late 1960s exhaustively investigating the affair, suggested that the agent had been asked "loaded" questions by Morton, who is known to have been working on the Riga report and had no doubt put the two together in his mind.

On 13 October SIS assured Sir Eyre Crowe that Morton's information provided "strong confirmation of the genuineness of our document (the Zinoviev Letter)". This was interpreted by Crowe as "absolutely reliable authority that the Russian letter was received and discussed at a recent meeting of the Central Committee of the Communist Party of Great Britain", and on this basis he recommended to MacDonald that a formal note of protest be submitted and full information be given to the press.

Morton's "strong confirmation", therefore, already perhaps more than the evidence supported, became "absolutely reliable authority", and the basis for explicit government action. It was only after the Soviet charge, Christian Rakovsky, had dismissed the letter as "a gross forgery" (which it almost certainly was) that on 27 October Crowe asked Malcolm Woollcombe for further information. Had, for example, the text been received in English or Russian and could an SIS officer explain things personally to the Prime Minister, who in the meantime had himself begun to wonder if the letter were bogus? Riga told Head Office that their original version had been in Russian, which had been translated by a secretary in the station before transmission to London, thus revealing that the English text was not quite as "authentic" as had at first been claimed....

On 12 November another committee, chaired by the new Foreign Secretary, Austen Chamberlain, and including Lord Curzon, was formed to investigate the matter. A week later, "after hearing all the necessary witnesses", they "were unanimously of the opinion that there was no doubt as to the authenticity of the letter". We do not know who constituted "all the necessary witnesses" (there is no written report of their deliberations), but they do not seem to have included anyone from SIS. Sinclair had evidently been ready to give evidence, for he provided Crowe with a note of "five very good reasons" why the letter was considered genuine which he had prepared in the event of being called before the committee. Sinclair declared, wrongly, that the letter had come "direct from an agent in Moscow for a long time in our service, and of proved reliability. He is an official in the Secretariat of the 3rd International, who works directly under Zinoviev and has access to his secret files." Though Sinclair may have believed this to be so, it was not precisely the case, as the claimed source was one of FR/3's sub-agents, about whom much was alleged but little definite was known. In his second and third reasons, Sinclair repeated some circumstantial corroboration, including the highly suspect assertion that the letter had been received by the Communist Party of Great Britain. Two further reasons turned round the possibility of the letter being a forgery. On the one hand, Sinclair baldly stated that "if it was a forgery, by this time we should have proof of it", which was more a matter of faith than evidence, and, on the other, he declared that "the possibility of being taken in by White Russians had been entirely excluded". SIS "made it our special business to be acquainted to the methods and personnel of the various White Russian and other forging organisations, especially the main one in Berlin (Orlov), with the object of preventing ourselves from having forgeries planted on us". In this particular case, moreover, he stated categorically that SIS was " aware of the identity of every person who handled the document on its journey from Zinoviev's files to our hands".

We might allow that Sinclair (or whichever SIS subordinate drafted the paper) had in mind some rather fine distinction between being "aware of" and "knowing" an identity, but in the sense which the assertion was clearly meant to convey to Crowe or the Foreign Secretary (or whoever), it was simply untrue, as FR/3 never revealed the specific identity of his alleged Comintern source. Only one of Sinclair's reasons, his fifth "because of the subject matter" - was actually any good at all, though this was still essentially circumstantial evidence. Sinclair correctly argued that the letter "was entirely consistent with all that the Communists have been enunciating and putting into effect", though he ignored other evidence which suggested that, at least temporarily, the Comintern had been anxious to avoid any action which would undermine MacDonald's minority Labour government. In the spring of 1924, for example, the Riga station had sent London a copy of a letter from the Comintern to the CPGB stating that overt anti-government action "was only permissible should the Government commit some grave infringement of the rights of the working classes".

SIS's resolute validation of the Zinoviev Letter, and its suppression of any evidence to the contrary, underpinned the consistent Foreign Office position for the next fifty years (at least) that the letter was genuine. Since the general content of the letter was never in doubt - the Soviets were indeed keen on fomenting revolution in Britain - and bearing in mind the broadly (and sometimes fiercely) anti-Bolshevik views held by SIS officers, among many other public servants, the Service attitude appears to have combined an element of wish-fulfilment with an understandable, if unattractive, desire not to admit to having made a mistake. SIS, and the security and intelligence agencies in general, have also been accused of leaking the letter both to the press and to Conservative Central Office in a deliberate effort to discredit the Labour government. "As you know the civil service has no politics," wrote one official in November 1924 to Lord Derby, a former Conservative Secretary for War, "but I fancy they would contribute heavily to a statue to Zinovieff & Mr. Campbell, for the effect they had on the election.''

So it may well have been in SIS, whose officers had numerous contacts and acquaintances in Conservative political and business circles. It is highly likely that some talk of the letter, if not the text itself, was shared beyond Whitehall. In April 1969 Desmond Morton even claimed that Stewart Menzies had sent a copy of the letter to the Daily Mail through the post, an assertion greeted with "amazement and disbelief" inside SIS.

The 1931 dispute with Special Branch over the running of agents in the United Kingdom was a case in point. While the Secret Service Committee essentially took Sinclair's side and primarily put the matter down to a personality clash, describing the policeman Colonel Carter as "temperamentally incapable of taking a broad view or of seeing that all three organisations (SIS, MI5 and Special Branch) were really working for the same cause," it is clear that both Carter and Special Branch had a case. Apart from Carter's alleged left-leaning politics (as identified by the politically right-wing Maxwell Knight and which may simply have reflected the policeman's anxiety to be politically even-handed), and beyond Sinclair's claims of the practical necessity of SIS operating within the United Kingdom, SIS had no formal brief to run agents at home. Carter, moreover, was not alone in resenting and opposing SIS's expanding domestic work, as he was backed up by the Metropolitan Police Assistant Commissioner Trevor Bigham. The continuation of domestic work, and the expansion of the Casuals, led directly to the embarrassing (for Sinclair) meeting in January 1931 when he was carpeted by Anderson, and following which, far from any unified organisation being created, SIS's remit was restricted, it was stripped of the Casuals, and MI5, as the Security Service, was given expanded responsibilities for domestic British counter-intelligence. One consequence, indeed, of Sinclair's persevering with SIS domestic operations was finally to torpedo any chance of the unified intelligence organisation he had so desired.

Of the first three Chiefs of SIS, Sinclair demonstrated the greatest tendency to cross the fine line between legitimate intelligence work, providing the government with clandestinely acquired information, as well as a political and military early-warning system, and becoming politically engaged in the policy-making process. During the Zinoviev Letter affair both Sinclair and his dynamic subordinate Desmond Morton (whom Sinclair loyally supported) asserted the genuineness of the letter rather more categorically than the evidence allowed. In part this was to protect the reputation of the Service, but it also reinforced a clearly anti-Labour political agenda.