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 in Co Antrim.
There were also calls for First Minister Peter Robinson and Deputy First Minister Martin McGuinness to "sort out the mess resulting from the process which they initiated".
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."
Dr Haass came to Northern Ireland to try to get some sort of settlement on the three contentious issues.
There was significant progress on dealing with the past, but the talks ended on New Year's Eve without any deal, especially on the issue of the flying of flags.
Sinn Féin said it was prepared to sign up to the proposals, while Mr Robinson's DUP said it was 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 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.