NO MP ever forgets their maiden speech in the House of Commons. In my case the memory was deeply reinforced by press coverage the following morning, with The Sun leading the press pack with a full front-page headline: "Red Ken smears heroes".

At Prime Minister's Questions that afternoon, Mrs Thatcher denounced my speech and then rounded on Neil Kinnock for not doing the same. Poor Neil made the fatal mistake of equivocating that "while the allegations are most probably not true...", thus prompting a Tory MP to claim that Kinnock sounded "like Klaus Barbie's defence lawyer". As Barbie was on trial for helping organise Hitler's genocide, this was a trifle over the top.

The allegations I made back in July 1987 read like a Frederick Forsyth novel. If true, they were a time bomb ticking away at the heart of Mrs Thatcher's government, involving her closest colleagues. But Labour's front bench wouldn't touch them with a bargepole.

Download the new Independent Premium app Sharing the full story, not just the headlines

Rumours began to circulate that Kinnock had been warned by MI5 that if he did pursue these claims, then damaging stories about Labour MPs' sexual and financial peccadilloes would be leaked to journalists. MI5 wasn't joking. Pictures of a married former Labour cabinet minister in the company of two extremely attractive Yugoslav women popped up in the gossip columns.

My allegations were that a small group of MI5 and MI6 officers conspired to wreck the cease-fire negotiated between the Labour government and the IRA in February 1975. Captain Robert Nairac led a group of loyalist paramilitaries across the border into Ireland to assassinate John Francis Green, a leading IRA figure. A few months later, in July 1975, Nairac again led loyalist paramilitaries, disguised in Ulster Defence Regiment uniforms, in an attack on the Miami Showband, who were at that time Ireland's most popular group. Three members of the band were killed and one of the guns used at the scene was the gun used to kill John Francis Green. When arrested, one of the loyalist paramilitaries was revealed to be a sergeant in the UDR.

Nairac was not at that time implicated in the incident, but the MI5 plotters were successful. The IRA reacted to the Miami Showband killings by slaughtering five people in the Bayardo Bar the following month, unleashing a wave of tit-for-tat killings that escalated until the Northern Ireland Secretary, Merlyn Rees, terminated the cease-fire on 12 November.

Making such allegations brought down upon my head a wave of condemnation, not just from the Tories and the press, but also from the Labour leadership. In the years that followed, I submitted more than 300 questions about the "dirty" war in Ireland and after years of denial, the government was forced to admit that the intelligence services had engaged in a covert black propaganda campaign, code-named Clockwork Orange.

Colin Wallace, a former Army intelligence officer who had been one of my two key sources of information, was found to have been framed for manslaughter and was eventually compensated. My second source, Captain Fred Holroyd, who worked for MI6, was discredited by being committed to a psychiatric hospital and has never received the compensation that is his due.

This has become relevant again because the Sunday Mirror has published letters purportedly from Nairac to Oonagh Flynn, the mother of his son. She recounts how Nairac told her he had killed Green and gave details of the Miami Showband killings. His letters also recount his fears that he is being set up: "I know they are shitting me up big-time, boxing me in." Some have claimed that an Army unit was operating near the Three Steps pub where Nairac was captured by the IRA, and did not intervene to save him.

It will be a simple matter to determine whether the handwriting in these letters shows that they were really penned by Nairac, and DNA tests could confirm Oonagh's claim that he was the father of her son.

Most of the key players in these events have died or are no longer in positions of power, but what they tell us about the operation of the security services is so important that Oonagh's claims must be investigated officially.

The current cease-fire and peace process in Northern Ireland hang on a thread in part because of republican suspicions that the security services are not impartial. The allegations of shoot-to-kill policies were investigated by the Stalker and Sampson inquiries but never published. This poisonous history bedevils Chris Patten's investigation of the RUC and the issue of whether it should be reformed or abolished. The way to create an Ireland at peace with itself is to create something like South Africa's Truth and Reconciliation Commission, examining the horrors of recent years irrespective of who committed them. Only then will it be possible for the people of Ireland to build a peaceful future.

It would also mean that we can finally pass judgement on Robert Nairac. When carrying out his orders, did he know they were issued by people trying to undermine the elected Labour leadership of the day? There is also the much more explosive issue of the trail leading to the outer office of Mrs Thatcher.

The spymaster Peter Wright, of Spycatcher fame, makes no mention in his book of the extensive work he undertook in Ireland, yet he was the central figure among the group of MI5 officers trying to bring down the Labour government. I believe that the most likely scenario is that Wright and others directed the murder of Green and the Miami Showband killings in order to deny the minority Labour government the popularity that would have followed from its concluding a peace deal with the IRA.

Wright met the former MI6 officer Airey Neave MP, who was not only Mrs Thatcher's shadow Northern Ireland minister but the head of her private office, her closest friend and the man who personally introduced her to MI5 and MI6 officers. Did Wright brief Neave on what was going on, to destabilise the Wilson government? Neave himself privately employed Colin Wallace to spread disinformation and black propaganda.

A full investigation could reveal that Airey Neave, the man who organised Thatcher's seizure of the Tory party in 1975, was also guilty of treason and an accessory to murder.