Thames Valley Police began to investigate Detective Sergeant Mark Kearney ostensibly as a way of silencing an officer they suspected of talking to the press.

In an effort to stop embarrassing titbits about the force appearing in the Milton Keynes Citizen, they bugged his car, raided his house and those of his friends and seized their belongings.

But in using a sledgehammer to crack a nut, Thames Valley Police did not foresee that their ill-conceived efforts would draw in ministers and MPs, lead to an official Whitehall inquiry and expose one of the most shadowy aspects of police work - the eavesdropping carried out on prisoners and their visitors in British jails.

Mr Kearney, a policeman for 30 years and the intelligence officer at Woodhill high-security prison, was charged with misconduct in a public office for leaking what the courts have ruled were “trivial” stories.

One concerned an MK Dons footballer brawling in a nightclub, while another, about day-release prisoners working for the Citizens Advice Bureau, appeared in a Sunday paper written by the journalist son of the Security Minister, Lord West of Spithead.

Far from leaking sensitive information, the officer said he had a long record of protecting it and sticking to the rules when senior police officers put pressure on him to bug inmates. He believes that his refusal to bend and break the rules on surveillance led to him being victimised.

“I think I became a bit of a thorn in their side. I became quite awkward,” Mr Kearney told The Times.

At an early stage in the case, Mr Kearney resigned from the police and decided that the only way he could keep himself out of jail was to reveal the extent of what he knew - but had never leaked - about high-profile prisoners and covert surveillance in the prison system.

What was disclosed in legal documents was devastating. It emerged that Mr Kearney had been asked to record conversations between Sadiq Khan MP, a former solicitor and now a junior minister, and his constituent Babar Ahmad, who is wanted in America on terrorist charges.

The revelation provoked a scandal and led to an official inquiry by Sir Christopher Rose, the Surveillance Commissioner. But perhaps more seriously it disclosed the existence of “talking tables” in prison visiting areas - where microphones are embedded to record inmates' conversations.

In documents seen by The Times, Mr Kearney says he was frequently asked to eavesdrop on legal visits between solicitors and their clients and always rejected the requests.

His defence papers said he had previously fought to keep secret the extent of prison surveillance. Mr Kearney's unit had recorded phone calls and visits involving Ian Huntley, the school caretaker who murdered Holly Wells and Jessica Chapman in Soham in 2002. As his trial approached, the Crown wanted to use the conversations in evidence but Kearney resisted, fearing this would alert prisoners to the use of bugging techniques in jails.

In one document, Mr Kearney's lawyers state: “The defendant believes it was Hazel Blears MP, then a Home Office minister, who directed the use of this material at the trial despite the fact that [Kearney] understood that the defence for Huntley were prepared to admit that the girls had been in his house.”

The court papers say that Ms Blears's alleged intervention undermined the use of surveillance in prison as an intelligence-gathering tool. The day after it was used at trial one inmate was recorded as he sat alone in a visiting room saying: “Hello officers.”

Pressure to conduct illicit recordings came, Mr Kearney said, from forces around Britain. The Metropolitan Police Special Branch was particularly keen, he said, to bug terrorist prisoners or covertly search their cells.

Mr Kearney said: “At Woodhill we were well respected because we produced a good product. We spent quite a lot of the State's money installing the equipment. Prisoners were moved from other jails to Woodhill so we could provide the product. We were victims of our own success.

“Officers often don't understand the difference between intelligence and evidence. The two are totally separate. A good case might use intelligence to lead to the evidence, but you don't expose the source or the methodology.” Mr Kearney was five weeks away from retirement when he was charged after an investigation in which Thames Valley officers bugged his car.

That device picked up 20 hours of conversations between him and Miss Murrer. The handful of stories that later appeared in the Milton Keynes Citizen were were deemed sufficient to pursue Mr Kearney and Miss Murrer through the courts. Charges were also pressed against Mr Kearney's son, Harry, a serving soldier, for telling a local reporter that Thames Valley Police had lost the keys to one of its stations.

Mr Kearney said: “To get at me the police have tried to bring my son down as well - we used to call it hostage taking, arresting a suspect's family to make him crack. But the Army have stood by him.”

The case has taken a toll on Mr Kearney's health. He was taken to hospital last week after falling ill in the dock. Doctors suspect a stroke.

“The biggest personal effect is that my life has been put on hold,” Mr Kearney said. “I wasn't going to stay in the police, I'd done my 30 years. When this is over, finally over, then I'll realise what the damage is.”

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