Northern Ireland's First Minister, Mr David Trimble, has given his strongest signal to date that he is not yet prepared to sit in government with Sinn Fein, because he is not convinced of the party's commitment to non-violence.
Mr Trimble's comments brought an implicit rebuke from the Deputy First Minister, Mr Seamus Mallon, who said there was a burden of duty on those who had signed the Belfast Agreement to honour it.
The Ulster Unionist Party leader told MPs at Westminster last night: "I have to say that as things stand today, in the circumstances that exist today, I have no confidence in the commitment of Sinn Fein to non-violence and to exclusively peaceful means."
Mr Trimble was speaking against the backdrop of mounting controversy and concern about the circumstances surrounding the murder of Mr Andy Kearney in the New Lodge in Belfast at the weekend, and as anti-agreement unionist MPs prepared a series of amendments to the Northern Ireland Bill seeking direct "linkage" between the issue of decommissioning IRA weapons and Sinn Fein's entry into the executive.
Four Ulster Unionist MPs - Mr Jeffrey Donaldson, Mr William Ross, Mr Roy Beggs and the Rev Martin Smyth - defied Mr Trimble in the Commons last night, joining forces with the DUP, the UKUP and four Conservative MPs in voting against the Bill on the Second Reading.
The Bill was given a second reading by 322 votes to nine, a government majority of 313.
Speaking during the debate on the Northern Ireland Bill, which will bring the Northern Ireland Assembly and other institutional arrangements established by the Belfast Agreement into being, Mr Trimble told MPs that, were he faced with the issue in the immediate circumstances, he would "have to move" to seek Sinn Fein's exclusion from office in accordance with the provisions of the Bill.
Looking to the future, Mr Trimble asked: "Will things change?" That, he said, was essentially in the hands of Sinn Fein.
"They know what they have to do in order for things to change. We have spelt that out clearly and in detail in our manifesto for the Assembly elections, so it's up to them. So far they have done nothing positive, nothing at all. They haven't even fulfilled the expectations they created in the media a few weeks ago about the disappeared."
The First Minister was heard in silence by dissident Ulster Unionist colleagues, but Mr Mallon said the legislation effecting the agreement was not negotiable and there was a burden of duty on all who had signed it to honour it on behalf of those co-signatories not represented in the house, on behalf of the Irish Government and on behalf of the people who had endorsed it in the referendums.
The DUP leader, the Rev Ian Paisley, warned: "Do not think you can ignore those who represent at least 50 per cent of the unionist population."
The anti-agreement unionists were facing an uphill battle as it became clear that the Conservative Party would not carry its reservations about the Bill to the point of opposition in the division lobbies. But in a further indication of the pressure on Mr Trimble, his deputy party leader Mr John Taylor said his support for the Bill at Third Reading was not guaranteed, as he called for assurances about the exclusion of parties inextricably linked to paramilitary organisations continuing to perpetrate violence.
MPs will consider the Bill in committee until next Monday, and the government hopes to complete all the Commons stages before the house rises for the summer recess on Friday week. The Bill goes to the Lords in October.