A joint investigation by the BBC and Washington's Insight magazine revealed that Dr Robin Pearson, a lecturer in economic history at Hull University, spied for 12 years for East Germany's notorious Stasi secret police.

The BBC's Graham Satchell : "Dr. Robin Pearson's home was deserted today"

The revelations about Dr Pearson come days after two other Britons were unveiled as former KGB agents - 87-year-old Melita Norwood and ex-Scotland Yard detective John Symonds.

A former KGB agent has also claimed this week two Labour MPs, Tom Driberg and Raymond Fletcher - both now dead - had been spies.

The BBC has discovered MI5 knew about Dr Pearson's role in 1994. A file was passed to the Crown Prosecution Service but they decided there was not enough "admissible" evidence to prosecute.

The Home Office said Mr Straw had been briefed about the case, by MI5, only last weekend.

Mr Straw is coming under increasing pressure to make a statement in the House of Commons about Britain's "forgotten spies".

Shadow Home Secretary Ann Widdecombe said: "I've been asking Jack Straw for over a week for a comprehensive statement.

"Every day there has been another development to this farce.

Chairman of the Commons Intelligence and Security Committee Tom King: "This is serious"

"Is Jack Straw going to finally make a statement or is he going to let the press do his job for him? That is the question."

Dr Pearson was recruited by the East German secret police, the Stasi, in 1977 and served as a "long-term penetration agent" for 12 years.

Robin Pearson has declined to comment on the allegations

When the German Democratic Republic (GDR) collapsed in 1989 hundreds of files were destroyed by the Stasi.

But four files survived, including one which contained information on a British Stasi agent codenamed "Armin".

He has now be revealed as Dr Pearson, a 44-year-old lecturer who is married with two children.

Anthony Glees: "I think he betrayed secrets"

Anthony Glees, an intelligence expert at Brunel University, said his information would have put his former colleagues in serious jeopardy.

A Hull University spokesman said there would be no disciplinary action against Dr Pearson, who is reportedly on holiday.

He told the BBC: "Whatever Dr Pearson has been doing in his private life is a matter for him.

"Our responsibility as an employer is to ensure he carries out his duties as teacher, researcher and an academic in a satisfactory way and there is no doubt that he has done so.

"There is no question of disciplinary action, but obviously we shall want to talk to Robin to get his side of the story."