MR David Trimble last night appeared to rule out substantive negotiations involving Sinn Fein until "the methodology" of decommissioning weapons has been agreed "and the process is begun".
The Ulster Unionist leader delivered his hard line message to the a British Prime Minister, Mr John Major, during nearly two hours of talks at 10 Downing Street.
Mr Trimble held out the prospect that the starting of a decommissioning process could be delayed until September, and that discussions about the agenda and procedures for negotiations could proceed in parallel with the work of a decommissioning sub committee until then.
But he would appear to have cast a huge question mark over the ability of British and Irish Ministers, meeting in London today, to agree a formula permitting the opening of "inclusive" all party negotiations in Belfast next Monday.
The Tanaiste, Mr Spring, will meet the Northern Ireland Secretary, Sir Patrick Mayhew, ahead of President Robinson's arrival today at the start of her four day official visit to Britain.
But Mr Trimble's tough stance has checked earlier confident predictions of an Anglo Irish breakthrough sufficient to persuade the IRA to reinstate the ceasefire and permit Sinn Fein's entry to the talks process.
The expectation had been that London and Dublin would finalise a process allowing decommissioning to be considered in parallel to political negotiations with progress on both fronts to be considered during a "review period" in September.
But Mr Trimble last night told The Irish Times he would not agree to discussions extending beyond the agenda and procedures for the negotiations until the method of dealing with the decommissioning of paramilitary arms had been settled.
And he said Mr Major had assured him he was determined that the arms issue would be settled during talks, "and that he would not allow the issue to be sidelined".
Sources last night suggested Mr Major and the Taoiseach, Mr Bruton, may have to become directly involved, as Ministers grapple with the aftermath of the North's election.
It is understood Mr Major and Mr Bruton are scheduled to attend the opening plenary session of the negotiations on Monday but the future of the entire process will remain in doubt unless Sinn Fein is present at the conference table.