Senior officers have spoken for the first time about how they trapped London’s most corrupt policeman with a listening device in his car.

Mesut Karakas was jailed for 13 years in 2010 for attempting to kidnap a bank manager in front of his family and force him to hand over cash.

But the bug revealed every detail of the plot and police moved in on his gang before they arrived at the bank manager’s house.

They were even heard discussing TV series The Wire and the “golden rule” not to openly talk about their plans.

Five years on, Detective Superintendent Chris Robson of the Directorate of Professional Standards — dubbed the Met’s “ghost squad” — remains baffled as to why Karakas went bad.

“He was a uniformed officer, a beat officer, and he moved around a couple of stations ending at Greenwich,” said Mr Robson. “There was nothing about him. I would not say he was a truly exceptional officer — pretty much your average police officer.”

He added: “It’s a very difficult set of circumstances to explain and I think it is something which will puzzle me for the rest of my service.”

By 24, Karakas had served four years as a Pc but was already suspected of links to two major drug dealers and Turkish organised crime in London. Doubts were raised in 2007 when there were suspicions he had injured himself with scissors to frame a suspect.

Then he and his gang launched a serious baseball attack on a man outside a pub in Islington. The following day Karakas took down details of the investigation from the police computer and bribes and threats were made to the victim to drop the charges.

Mr Robson said: “When (allegations) are made it’s extremely important they are taken seriously so we employed a number of covert tactics and part (of that) was planting a probe in Karakas’s car — a listening device.”

They spoke of what they would do to the bank manager and his family — “will the female scream?” — and set aside 90 minutes to make him co-operate in full. The five men planned to stage a roadworks scene near the victim’s home as a distraction for the kidnap.

When they moved in police uncovered false registration plates, industrial gaffer tape, dust masks, a balaclava, plasti-cuffs, industrial ear protectors and the van to be used in the plot.

“We heard them planning the route, what time they should commit the offence when there would be less police on the street and what CCTV cameras would be looking at them,” said Mr Robson on Channel 5 programme Breaking The Law: Bent Coppers.

“I was fully aware that there was a real chance of violence being involved. The victims would have their mouths taped over and hands tied and people with balaclavas would be coming through their door.

“I didn’t let them get near the victim as the consequences could have been far too grave. The people he committed the offence with are nothing to do with Turkish organised crime — they were his friends from schooldays. They were clean skins, they had no previous criminal convictions against them.”

Karakas pleaded guilty at Blackfriars crown court to assault, conspiracy to kidnap and misconduct in public office. Three members of his gang also received major sentences.

Breaking The Law: Bent Coppers will be broadcast at 9pm tomorrow on Channel 5.