A former British Secret Service MI5 agent who blew the whistle on an attempt to bug the Pakistani high commission in London has fled to Europe and is threatening to reveal more about the aborted operation.

The agent claimed to have a dossier of evidence, including documents and photographs revealing details about MI5's espionage techniques and what information it was trying to obtain about Pakistan, The Sunday Times reported.

The man, a building contractor who was codenamed Notation, left Britain in November, shortly before the Sunday Times exposed the spying operation.

British Home Secretary David Blunkett, who is furious about the botched spying operation and the leak, has denied signing warrants authorising the espionage attempt.

That appears to contradict claims by Notation who said he received detailed assurances from MI5 that the home secretary has issued warrants. "Claire, my handler, told me on several occasions how MI5 obtained the warrants from Blunkett," he said.

Notation is now being sought for questioning. MI5 wants to prevent him revealing further secrets. A home office source said he might face prosecution under the Official Secrets Act.

Notation was recruited by MI5 in 2001 to provide gain to the Pakistani high commission in Belgravia, London during restoration work.

Posing as building workers, members of MI5 entered the building, stole codes for sending secret messages and planned to plant listening devises and remove documents.

The operation was aborted after Notation became worried that his role could be exposed. He later wrote to the parliamentary intelligence and security committee, saying he was concerned for his safety.

The affair has caused a diplomatic rift between London and Islamabad.

The opposition Conservatives Party is demanding a House of Commons statement from Foreign Secretary Jack Straw, claiming the botched operation had severely damaged Anglo-Pakistani relations.

Pakistan said it had sent its own experts to debug the high commission. "They are in Britain," Foreign Minister Khurshid Kasuri told a press conference. He had demanded an explanation for the spying during his meeting with Straw in London during his visit to the UK. Straw put him off saying he was forbidden by law from commenting on intelligence matters.

Kasuri said there is now "a question of confidence" between the two countries.