Ulster Unionists will not re-enter an assembly which includes Sinn Fein, party leader David Trimble has said.
David Trimble launched a scathing attack on the DUP
He was speaking at the annual meeting of the Ulster Unionist Council, being held in Belfast.
Mr Trimble said if asked by Sinn Fein's Gerry Adams how to restore unionist faith in the Executive, he would say he had no idea how this could be done.
The Ulster Unionist leader also used his speech to launch one of his strongest attacks yet on the DUP.
He said the DUP had given republicans a "green card" to continue criminal activity in the failed political negotiations before Christmas.
"No wonder republicans thought they could get away with the Northern Bank raid," he added.
Speaking about the possibility of an electoral pact with the DUP, Mr Trimble said his party would not be driven off the political battlefield.
He warned the DUP that if they contested the South Belfast constituency in the next general election and handed the seat to a nationalist they would be "condemned as anti-unionist".
"We can fight at local government level, but Westminster seats must not be thrown away," he said.
"The man in the street can see that there are unionist seats that could be lost to nationalists and republicans on a split unionist vote."
Mr Trimble said the DUP had failed to get decommissioning and had conceded to an all-Ireland Assembly of MLAs and TDs.
"They also conceded a structured role for northern nationalist MPs and MEPs in the Dail, an immediate deal on the devolution of policing and justice, and turned a blind eye to the release of the killers of Garda McCabe," he added.
He also said the DUP should join with his party and "other democrats" to ensure there would be no deviation from the constitutional settlement enshrined in the Agreement.
He said: "I believe that it (the Agreement) should unite unionism.
"I ask the DUP and others to support it, and I have to say to them that they would do better to support this than to continue to send signals to republicans that they are just waiting until after the elections to get back into bed with them."
North Belfast MP Nigel Dodds described Mr Trimble's remarks about his party as "pre-election posturing".
"The DUP will not take lectures from the UUP on giving away concessions to republicans," he said.