Last night, Professor Christopher Andrew of Cambridge University, the secret service historian who identified the 87-year-old great-grandmother Melita Norwood as a Soviet agent, said: "One of those named will be a prominent public figure who is now dead."
The disclosures will increase pressure on the Government, which is facing intense criticism for not prosecuting Mrs Norwood, who was unmasked yesterday as a Soviet spy of 40 years' standing and the KGB's most important catch. Jack Straw, the Home Secretary, yesterday asked the director general of MI5, Stephen Lander, to prepare a full report on the case. The Home Office insisted he had not been involved in the decision not to prosecute.
Earlier this year, John Morris, then Attorney General, had been made aware of Mrs Norwood's identity, but decided that "it was by then too late to undertake a prosecution".
At a press conference outside her home yesterday, Mrs Norwood said: "I did what I did not to make money but to help prevent the defeat of a new system which had, at great cost, given ordinary people food and fares they could afford, a good education and a health service." Asked if she regretted her actions, she said: "No, no, no."
The identity of Mrs Norwood and the other spies is contained in a huge archive of files brought out of Russia by a defector, Vasili Mitrokhin, in 1992. He smuggled out six trunks of files from the KGB's most secret archive.
According to Professor Andrew, the files identify KGB agents and operations in virtually every country outside the Communist bloc.
Tom King, the former Tory Secretary of State for Defence, and now chairman of the Parliamentary Security and Intelligence Committee, said the unmasking of Mrs Norwood was part of "the most complete and extensive intelligence ever received from any source". He said that there would be further revelations in the coming days.
A British intelligence officer agreed yesterday that further major KGB spies would be revealed. ``There could be up to 12 more names to come out. You have to spend weeks, sometimes months, painstakingly going through file after file checking codenames and places then trying to match them with the names of real people who are either still living or existed."
Some of the files deal with KGB agents in Britain who were active right up to the end of the Cold War. Mrs Norwood, codenamed "Hola", is herself believed to have helped to recruit agents for the KGB. In 1967 she allegedly helped the agency recruit a civil servant who was codenamed "Hunt". Through the 1970s he was a paid agent and provided information on British arms and arms deals.
The imminent publication of the Mitrokhin archive has sent shockwaves through Whitehall, with departments rushing to wash their hands of the matter. Questions will now be asked about how much ministers knew, and whether the director general of the security services decided not to inform them of the contents of the archive.
Mr King said his committee would conduct a thorough investigation into the documents, and also into the handling of the case by both Conservative and Labour administrations. "Questions need to be asked about who knew what and why there were no prosecutions," he said.
Michael Howard, Mr Straw's Conservative predecessor, said he had "no knowledge" of the case. Kenneth Clarke, Home Secretary from 1992 to 1993, and Kenneth Baker (1990-92) will also be quizzed.
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