A retired Special Branch officer has revealed how he helped nab Sinn Fein bomber Gerry Kelly along with Marion and Dolours Price after the 1973 London bombings — thanks to an IRA mole.

George Clarke, who retired from the RUC in 1986 and is now 72, was tipped off by a veteran republican terrorist that Kelly and co were plotting to bomb the English capital.

George’s source ‘Seamus McMahon’, a senior member of the Provisional IRA and one of the so-called godfathers, informed him of the attacks just 14 hours before the bombs went off.

In his book Border Crossing, George tells a boss: “He states that at 12 noon tomorrow four car bombs will go off in London.

“He is only sure of two targets, the Old Bailey and New Scotland Yard. He has named some of the bombing team of men and women.

“Gerry Kelly of the Belfast Brigade would appear to be in charge of the operation. There are also two sisters named Price taking part.

“The cars were driven over with the bombs built into the car boots. They had a training camp in the South from where the operation was planned.”

With the help of George’s intelligence, hundreds of plain-clothes cops were deployed to search London but they were unable to prevent Kelly and his team causing death and injury.

Two of the car bombs were defused but the other two exploded. One person was killed and almost 200 people were injured.

But George was able to get the terror gang caught at Heathrow Airport — despite fears it might compromise his informer.

He said: “The source later told me that he had worried like hell but in the end he was happy at the way things turned out.

“He was sad that Dolours and Marion Price had destroyed their lives, but was happy that Gerry Kelly was detained.

“He described him as ruthless and an important officer in the Belfast Provisional IRA hierarchy. ‘If he hadn’t been caught in London he might have done damage elsewhere, you are lucky to have him locked up’.”

Kelly, then aged 19, and eight others were found guilty of various charges relating to the bombings. Kelly was convicted of causing explosions and conspiracy to cause explosions and received two life sentences plus 20 years.

George said: “Who would have thought that on his release from prison Kelly would keep his deep association with Provisional Sinn Fein and, after the ceasefires, would eventually become an MLA at Stormont?”

Elsewhere in the book George reveals how a surveillance device he loaned to a garda friend made its way to former Taoiseach Charlie Haughey, who refused to return it and instead used it in his own office.

Other stories include how he introduced the Queen’s cousin, Lieutenant David Bowes Lyons, to an IRA source and remembers his friendship with IRA victim Captain Robert Nairac — who once babysat his children.

Border Crossing, by George Clarke and published by Gill & MacMillan will be on sale from Friday, October 16, priced £12.99.