New London Mayor Sadiq Khan was bugged by a Milton Keynes police officer when he visited a suspected terrorist in Woodhill Prison.
The now-retired officer, Mark Kearney, was ordered by Special Branch to secretly record conversations between MP Mr Khan and his childhood pal Babar Ahmad.
Mr Ahmad was being held in the prison’s high security unit on suspicion of running a website supporting Osama Bin Laden and soliciting funds for the Taliban.
He was later extradited to America and jailed for cyber terrorism.
Now Mr Kearney has revealed details of bugging in his book, All For Nothing.
He describes how human rights lawyer Mr Khan visited his imprisoned friend shortly after he had been elected MP for Tooting in 2005.
The secret recording equipment was “poised and waiting”, hidden under the table in the visiting room. But the resulting tape was nothing but mundane.
“Mr Khan told him about the elections. Babar Ahmad in turn congratulated his friend on his recent success,” states Mr Kearney.
“It was a very normal conversation between two old friends..There wasn’t anything in the recording that was subversive in the slightest.”
More than a year later, in June 2006, Mr Khan visited Babar again. One more, Mr Kearney was told to bug him.
“The conversation related only to the quality of Babar Ahmad’s legal team,” he said.
Mr Kearney questioned the legality of bugging an MP, but a government enquiry later determined that it was not against the rules.