The Ulster Unionists have rejected proposals by Dr Richard Haass to find a settlement on flags, parading and dealing with Northern Ireland's troubled past.
A week after the negotiating process ended in Belfast without agreement, the party last night said the final text presented to all sides was not viable and therefore unacceptable.
The decision followed a meeting by the party's executive in Templepatrick, Co Antrim, after which there were calls on the First Minister Peter Robinson and the deputy First Minister Martin McGuinness to "sort out the mess resulting from the process which they initiated".
A statement from the UUP said: "The Ulster Unionist Party seeks a positive resolution to the issues of flags, parading and dealing with the past.
"Consequently, this Executive believes the Haass document is not viable and therefore unacceptable.
This Executive further calls on OFMDFM (Office of the First Minister and Deputy First Minister) to sort out the mess resulting from the process they initiated.
"The Ulster Unionist Party will examine any measures brought forward by the First Minister and Deputy First Minister and report again to the Executive at its next scheduled meeting in February."
Dr Haass came to Northern Ireland to try to get some sort of settlement on the three contentious issues, and even though there was significant progress on dealing with the past, the talks ended on New Years Eve without any deal, especially on the issue of the flying of flags.
Sinn Fein said they were prepared to sign up to the proposals, but Mr Robinson's Democratic Unionists said they were not in a position to go ahead but he denied the process was a failure.
The outright rejection by the smaller Ulster Unionist Party heightened fears that this particular initiative is highly unlikely to go anywhere.
Yesterday Dr Haass said the draft agreement would leave people in Northern Ireland considerably better off. He and his talks vice-chairman Dr Meghan O'Sullivan published a two- page summary of their plans in a clear effort to get public support.
At the same time Mr McGuinness said they should be implemented as they stood, and urged other political leaders to show leadership.
He declared: "Richard Haass has delivered his final text. This is the time we need political leadership.
"The only purpose in establishing an all-party working group is to ensure the implementation of the document as it stands, not to reopen negotiations on its contents."
The Haass process was established in July to deal with what have become three of the primary obstacles to meaningful reconciliation.
Tensions over contentious parades regularly erupt into street violence while disputes over the flying of flags - on public buildings and in loyalist and republican neighbourhoods - cause community conflict.
Arguably the most complex issue has been how Northern Ireland deals with the legacy of a 30- year-conflict, with opposing sides retaining competing narratives of what happened and victims demanding truth and justice after more than 3,000 unsolved murders.
The document instead envisaged the setting up of the Commission on Identity, Culture and Tradition to examine the flags problem over a longer time frame - potentially 18 months.
Dr Haass recommended the replacement of the UK Government-appointed Parades Commission with a new devolved mechanism for adjudicating on contentious events.
This would consist of an administrative arm - the Office of Parades, Select Commemorations and Related Protests - to deal with applications to march and protest and potentially facilitate mediation between groups.
It would also see the creation of the Authority for Public Events and Adjudication - an independent regulatory body, chaired by a legal figure, which would deliberate on applications for unresolvable parading disputes.
Like the Parades Commission, it would have seven independent members, but the new authority would also provide more scope for appealing against decisions.