Mr Smith denied having any links with the Young Communist League or the Communist Party, even though he was an active member and branch secretary of a Surrey Communist group. When he realised the MoD knew of his past, he suddenly 'changed tack', admitted his Communist background and congratulated officials on their efficiency in discovering it.
Sir Derek Spencer QC, the Solicitor-General, for the prosecution, has alleged that Mr Smith was a dedicated Communist who was recruited by the Soviet Union 20 years ago, and passed on details of secret weapons systems while working for the electronics company GEC.
The trial, which was held in open court yesterday for the first time in three weeks, heard evidence from a man referred to as Mr D, who spoke from behind a screen. Mr D said he worked for the Ministry of Defence and interviewed Mr Smith at his own request in 1980. Mr Smith was then working for Thorn EMI, where he was transferred from classified to medical projects when his security clearance was withdrawn.
Asked if he had any links with the YCL - the Young Communist League - Mr Smith had affected not to know what the initials stood for. Mr D said that when he explained, Mr Smith said that 'as far as he knew he had never been a member of any political group at any time unless someone had put his name on a piece of paper'.
When he urged Mr Smith to tell the truth because he would not be prosecuted for telling lies, 'he suddenly changed tack and said 'All right, I admit it, I was involved in Communist Party activities'. He explained he had bought the Morning Star, the Communist Party newspaper, and was attracted by its analysis of the country's economic problems, particularly unemployment.'
Mr Smith told him he had been an active member attending party congresses but had become disillusioned 'by the party, the sterility of its arguments, by the very poor human rights record of the Soviet Union and the tyrannical way many Eastern Bloc states were governed'.
He said he left the party in late 1975 or early 1976, but when told by Mr D that the MoD knew it was a month before he joined EMI in mid-1976, he denied this was 'expediency' because of his new job.
The Surrey University electronics graduate went on to request the return of his security clearance because he said the best career opportunities in his field - quality assurance - were in classified work.
Mr Smith, 44, of Kingston upon Thames, south-west London, denies four charges under the Official Secrets Act. Sir Derek has alleged that Mr Smith passed on sketches and plans of Britain's secret weapons systems to his Soviet handlers at first for ideological reasons, but later for money.
The trial continues today.