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.
Mr Clarke said he was told by MI5 to offer his republican mole £500 to leave the explosives in the property on a Friday night.
It was to detonate the next morning, when it was known a number of high-profile IRA men visited the office to pick up weekly payments from party funds.
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, which was a lot of money in those days.
“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.
A spokesman for the UK Home Office said the security services did not comment on past intelligence operations.
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.
Though it details a dark period in Irish history, there is room for some lighter anecdotes in the book.
One story centres on Special Branch listening devices that former Taoiseach 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 Fáil 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.”
* Border Crossing: True stories of the RUC Special Branch, the Garda Special Branch and the IRA Moles is published this week.