THE violent death of an IRA man turned best-selling author has put the Northern Ireland peace process under renewed strain, with the IRA accused of breaking their ceasefire.

Eamon Collins, an outspoken former IRA ''supergrass'' who had angered the republican leadership, was found dead with ''very severe head injuries'' outside Newry, County Down early yesterday morning.

His body was discovered in a remote country lane at Doran's Hill by a couple walking their dog.

Hours later Tony Blair and William Hague clashed in the Commons over the continuing release of terrorist prisoners in Northern Ireland despite the lack of progress on decommissioning illegally-held weapons.

Rejecting calls to halt the releases, the Prime Minister said the wider interests of peace in the province demanded that paramilitary prisoners had to gain their freedom.

If an IRA link with Collins's death is proved, Sinn Fein will be barred from taking its seats in the Executive of the new Northern Ireland Government.

Collins angered the IRA by informing on his colleagues while in police custody in 1985, and then by ''going public'' and speaking out about the Provos on television and in his 1997 book, Killing Rage. In the past two years, his house in South Armagh was attacked, and he was injured in a suspicious hit-and-run accident blamed on the IRA.

Collins joined the IRA in 1979, acting as an intelligence officer. His information led to five murders, including that of Ivan Toombs, his superior in the Customs and Excise service.

The area where the dead man was found is renowned for its militant republicanism. The IRA's South Armagh brigade, which was sceptical about the Good Friday Agreement, is based nearby. In the early 1980s Collins gathered intelligence for that Provo unit.

His role led to his being convicted for conspiracy to murder Toombs and four others, but his sentence was reduced after he informed upon his IRA colleagues. Although he later retracted his evidence, the IRA never forgave him. Informing carries a death sentence, and, to make matters worse as far as the IRA are concerned, Collins did not disappear into exile, but stayed in South Armagh, going public with his story and his criticisms of the IRA.

When the RUC examined his body yesterday, his head wounds were so severe that his face was unrecognisable. Initially he was thought to be the victim of a hit-and-run accident.

Ulster Unionist MP Jeffrey Donaldson said last night: ''If the Provisional IRA was responsible, this is very clearly a breach of the terrorist ceasefire and the Government must face the consequences and take the necessary action against the parties involved.''

Sinn Fein chief negotiator Martin McGuinness offered sympathy to Collins's family, but said: ''Eamon Collins made an awful lot of enemies in the course of recent years.'' He added: ''Who was responsible for his death, I have no idea whatsoever.''

Collins was on a regular morning walk when he was attacked, according to his London-based solicitor. Mr Jason McHugh admitted: ''He was a realist, he realised the dangers and the inevitability that if you talk out against certain organisations you are going to have to watch over your shoulder.''

The killer or killers could have been associates of Thomas ''Slab'' Murphy, until recently the Provos' chief of staff. Collins gave evidence against Murphy last year when the latter unsuccessfully sued the Sunday Times for accusing him of IRA activity.

Collins was paid #10,000 by the Sunday Times. Murphy was humiliated, faces huge legal costs, and was replaced as chief of staff at a recent IRA army convention.

The next suspects are dissident Republicans, those associated with the Real IRA, who killed 29 people in last August's Omagh bomb, or the Continuity IRA, the only Republican group not officially on ceasefire.

The timing of the killing leads suspicions in this direction. At delicate moments in the peace process dissident IRA members have tried to undermine Gerry Adams by planting bombs.

The peace process is under real threat. Unionist leader David Trimble is adamant that he will not enter an executive (cabinet) with Sinn Fein until the IRA starts to decommission arms. Sinn Fein says it is not the IRA and anyway there is nothing in the agreement which says the IRA has to disarm before an executive is formed.

There is also the apparent upsurge in ''punishment'' beatings and shootings, which Northern Ireland Secretary Mo Mowlam condemned yesterday as ''barbaric acts'' during a Commons debate.

The debate ended with a Tory motion calling for a halt to the early release of terrorist prisoners until violence ceased in Northern Ireland being defeated by 343 to 141, a Government majority of 202. An amendment by Ministers calling on all parties to use their influence to bring punishment attacks to an immediate stop was passed without a vote.