On 27th April 1939, Parliament passed the Military Training Act. This act introduced conscription for men aged 20 and 21 who were now required to undertake six months' military training. On the outbreak of the Second World War, Parliament passed the National Service (Armed Forces) Act, under which all men between 18 and 41 were made liable for conscription. It was also announced that single men were called up before married men. The registration of all men in each age group in turn began on 21st October for those aged 20 to 23. By May 1940, registration had extended only as far as men aged 27 and did not reach those aged 40 until June 1941.

Provision was made in the legislation for people to object to military service on moral grounds. Of the first batch of men aged 20 to 23 and estimated 22 in every 1000 objected and went before local military tribunals. The tribunals varied greatly in their attitudes towards conscientious objection to military service and the proportions totally rejected ranged from 6 per cent to 41 per cent.

By the end of 1939 over one and a half million men had been recruited into the armed forces. Of these, 1,128,000 joined the British Army and the remainder were equally divided between the Royal Navy and the Royal Air Force.

The easiest way to avoid conscription was to ignore the summons to register for National Service. As a result of a shortage of people to enforce attendance, this method of avoiding the joining of the armed forces was highly effective. Another method was to hire a man who had already failed his medical, to impersonate you in front of the medical board. Jack Brack was rejected as unfit for service because of an enlarged heart. A few months later it was arrested and charged with impersonating eight different men at military medical boards. It was discovered in court that one man, a master tailor, had paid Brack £200 (£8,000 in today's money) for this work.

There was also a good market in buying forged medical discharge certificates. In May 1940 the police in London was investigating four gangs selling these certificates. Some doctors were willing to issue false medical certificates to friends and relatives. An investigation carried out by the General Medical Council resulted in several doctors being struck off for "infamous conduct". Other did it for profit. One doctor from London was found guilty of charging a man £367.10s. (£14,700) for his certificate. Dr. William St. John Sutton of Stepney, developed a scheme of selling certificates exempting men from duty. When he was arrested he was found with 700 forged certificates.

Desertion from the armed forces was a common problem. At one stage in the war there were over 24,500 men who were wanted for desertion. At the end of 1941 the government ordered a "round-up" of deserters. When police raided a Plymouth funfair they discovered that almost two-thirds of adult males checked did not have identity cards. However, before the men could be arrested someone let off a smoke bomb and they all escaped.

Deserters often resorted to crime in order to survive without identity cards or ration books. One of the most shocking crimes committed be deserters during the war was looting from bombed houses. In the first eight weeks of the London Blitz a total of 390 cases of looting was reported to the police.

The Lord Mayor of London suggested that notices should be posted throughout the city, reminding the population that looting was punishable by hanging or shooting. However, the courts continued to treat this crime leniently. When a gang of army deserters were convicted of looting in Kent the judge handed down sentences ranging from five years' penal servitude to eight years' hard labour. Some critics pointed out that Nazi Germany suffered less from this crime as looters were routinely executed for this offence.

The problem of desertion became worse when soldiers knew they were about to be sent abroad. Official figures show that large numbers of men due to take part in the D-Day invasion deserted. Between 6th June 1944 and 31st March 1945 36,366 of these soldiers were arrested by the Military Police, of these, 10,363 were charged with desertion.

The problem of desertion continued after the war. On 29th March 1950 Emanuel Shinwell, the Minister of Defence, announced in the House of Commons that there were still 19,477 absentees: 1,267 were from the Royal Navy, 13,884 from the British Army and 4,366 from the Royal Air Force.