Ulster Unionist leader David Trimble has threatened to walk out of the review of the Good Friday Agreement unless action is taken over the alleged false imprisonment of a man.
Mr Trimble said his party would walk unless Sinn Fein was excluded
Mr Trimble wants Sinn Fein excluded from the process over the incident involving dissident republican Bobby Tohill in Belfast last Friday.
Chief Constable Hugh Orde said on Saturday that members of the Provisional IRA were behind the incident.
Speaking in the Commons on Wednesday, Mr Trimble said the government's reaction to the incident so far was "rank moral cowardice".
"It is utterly unreasonable to expect us to remain in discussions with these people in these circumstances," he said.
"And I have to tell you that unless you can summon up the courage to act on
this matter within the next few days then I and my colleagues will take steps
next week to bring this process to an end."
DUP leader Ian Paisley said he had asked for an urgent meeting with Prime Minister Tony Blair about the incident, as well as a meeting with the chief constable.
He also said Mr Trimble's comments were "weak and feeble" and "simply will not wash".
"The UUP refused to support the DUP in our attempts to have Sinn Fein/IRA excluded from the executive. Mr Trimble cannot change his tune now without people noticing the difference," he said.
Mr Blair has warned that if the IRA is implicated in the incident "action will follow".
The prime minister told MPs that Sinn Fein and the IRA could not be allowed to talk about human rights one day and then start "beating human rights out of people the next".
Mr Blair said action will have to follow if the IRA are implicated
Meanwhile, the republican newspaper, An Phoblacht, said it had been told by a source speaking for the IRA leadership that the organisation did not authorise any action against Mr Tohill.
Four men have been charged in connection with the incident in Belfast.
The Irish Prime Minister, Bertie Ahern, said he would be raising concerns about the incident with Sinn Fein at a meeting with a party delegation in Dublin on Wednesday.
After the meeting, Sinn Fein's Martin McGuinness was critical of the Irish Government's comments about the alleged incident in Belfast.
Mr McGuinness said the Irish Government should "not be lining up with anti-Sinn Fein and anti-peace process elements".
The Alliance Party has requested a meeting with the four member commission tasked with monitoring paramilitary ceasefires to get it to produce a report on Friday's alleged incident.
Party leader David Ford added: "The public expects all parties to get on with the job of restoring devolution, but that is difficult if the government refuses to deal
with the issues with the urgency they deserve."
On Tuesday, Secretary of State Paul Murphy said the alleged incident was "a serious breach of paragraph 13 of the joint declaration".
This part of the document, produced last year as an attempt by the British and Irish Governments to move the political process forward, demands an end to paramilitary activity.
Mr Murphy said the Independent Monitoring Commission, which monitors paramilitary activity in the province, had been asked to investigate the incident and would produce a report on 1 May.
The political institutions in Northern Ireland were suspended in October 2002 amid allegations of IRA intelligence-gathering in the Stormont government.