The British Security Services plotted to blow up Sinn Fein's Dublin headquarters during the height of the Troubles, a former undercover police officer has claimed.

An IRA informer was asked to place the bomb in the offices on Kevin Street in the summer of 1971, according to George Clarke, a retired Royal Ulster Constabulary Special Branch Detective Sergeant.

The counter-terrorism officer, who left the police more than 20 years ago, said the security services eventually decided not to go through with the attack, but not before he had put the proposition to his source.

He made the claims in Border Crossing — his newly published memoir of his time working in the RUC's intelligence unit.

“I was approached by one of the men from the security services who asked if the source would be willing to do a job,” said the retired detective, who is now in his late 70s.

“It would involve leaving a bag in the Kevin Street office. He would get £500 for it. It was to be timed for a Saturday morning, that's when quite a few of the boys (IRA) would be there to pick up their 10 or 20 quid.”

Mr Clarke, who spent more than 40 years in the police in Northern Ireland, describes in the book the unease he felt over what was being suggested.

“This wasn't only agent provocateur, it was conspiracy to murder,” he said.

“I wasn't happy to say the least.” In the event, the alleged plot never got beyond the planning stages. Two weeks after it was first suggested to him, Mr Clarke said he was called to another meeting with the shadowy MI5 agent.

“He said they had changed their minds about it and just to forget it was mentioned,” he recalled.

The story is one of many recounted in the officer's 250-page account of his time working in RUC Special Branch during the worst of the Troubles.

Another involves a republican source presenting himself out of the blue to pass on a tip that led to the arrest in 1973 of current Stormont Sinn Fein Junior Minister Gerry Kelly at Heathrow airport, hours after he had carried out a bomb attack on London's Old Bailey.

“He (the informer) went to the guards (Irish police) in the south and wanted a lot of money for the information,” he said.

“But the guards weren't going to pay him thousands for information about London — if Dublin was the target that would have been different.

“So one of the guards told him to take a trip over the border and get in touch with Special Branch. He did that and I was on call the night he rang. He said London would be hit at noon next day. While we didn't have enough information to stop the attacks, we did know they would be leaving from Heathrow.”

Mr Clarke, who now lives in Scotland, said the episode is just one example of how undercover officers on both sides of the border co-operated with each other at a time when their commanders would not even speak to one another.

This secret link between the RUC and the Garda in the early 70s is a central theme running through the memoir.

“In and around 1971 there wouldn't have been a phone call between forces north and south, not one, no contact at all, even from Newry to Dundalk — they wouldn't even call each other about ordinary crime,” said the ex-officer.

“Things were getting hot and heavy (in regard to the violence) in those days so I decided to go and look for my counterpart across the border. I found him and after a few meetings and a few pints we started to work together.

“So there was this clandestine relationship during those critical years before internment.

“Many of the successes we had can never be told, but we thwarted countless attacks, we were having successes on a weekly basis in those days, either intercepting weapons or explosives.

“The public don't know what went on behind the scenes in the dark days of (the) 70s, but I've no doubt we saved hundreds of lives, maybe even more.”

Though it details a dark period in Irish history, there is room for some lighter anecdotes in the book.

One centres on Special Branch listening devices that former Irish premier Charlie Haughey got his hands on and refused to give back.

Mr Clarke had lent the bugs, which were hidden in pens and plug adapters, to a colleague in the Garda to have a look at.

The officer had in turn shown them to the then Fianna Fail Taoiseach, who apparently started to use them to great effect in his Dublin offices to listen in on conversations between other politicians.

“I kept on at this guard to get the devices back, but he said Haughey just refused,” he explained.

“He told me he had got angry with him on one occasion and said 'I need them back, they don't belong to me'.

“But Haughey said he couldn't have them, that they were so useful to him that they had changed the face of Irish political history.

“Well to that the guard responded: 'Well I hope you realise that it's the RUC Special Branch that's changing the face of Irish political history then!' “

Border Crossing: True stories of the RUC Special Branch, the Garda Special Branch and the IRA Moles is published this week by Gill MacMillan.