Ian Paisley's Democratic Unionists let republicans off the hook at intensive political talks, Ulster Unionist leader David Trimble has said.
David Trimble addressed delegates at his party's conference
Mr Trimble said the year since the assembly elections had been wasted, with no sign of the new deal promised by the DUP.
He said if Northern Ireland's devolved assembly was to return, Tony Blair must nail down the IRA on a commitment to fully disarm and end paramilitarism.
Addressing his party's conference on Saturday at Newcastle, County Down, he said the prime minister had missed an opportunity at the Leeds Castle talks in September to find out exactly what republicans were prepared to do to revive power sharing.
Mr Trimble said he suspected that Sinn Fein was "unable to persuade their grassroots to make what they would regard as big sacrifices".
"Like other parties, we do not know what republicans supposedly offered to Blair. I suspect the offer was more a bluff than anything else.
"Blair should have nailed it down but, with characteristic optimism, he rushed at it.
"The DUP could have covered themselves by confronting republicans and insisting they give clear details. But rather than engage in serious negotiations, they hid behind other issues.
"I did warn the DUP that they were letting republicans away in the smoke. Unfortunately, they did not listen.
"But that should not obscure the fact that the main responsibility lies with the government and republicans."
At the conclusion of intensive political talks at Leeds Castle in Kent, Mr Blair and Taoiseach Bertie Ahern said the thorny issues of IRA disarmament and future paramilitary activity appeared to be resolved.
However, the two governments were unable to get the Northern Ireland Assembly parties to sign up to a deal over power-sharing after unionists and nationalists clashed over future devolved institutions.
Mr Trimble said on Saturday that the negotiations which followed September's talks had "run out of steam".
"It is said that the government is preparing a paper to put before the parties. We have advised the government that they first nail down republicans," he said.
"There must be genuine acts of completion that satisfactorily resolve decommissioning and paramilitary issues.
"Without that prospect, there will be no progress. With it, there is something to do and we will be ready."
Mr Trimble said the DUP had not put forward any new deal during the negotiations and had in fact signed up to the Good Friday Agreement after six years of opposing it.
Mr Paisley's meeting with Mr Ahern in Dublin was "the most dramatic reversal of all", he said.
Mr Trimble also attacked the DUP for seeking separate elections in the assembly for the Stormont First and Deputy First Ministers if devolution returned.
"What is the gain if the DUP is not required to vote for a Sinn Fein DFM but it is prepared to accept a Sinn Fein DFM voted in by other means?" he asked.
"This is merely stripping out one of the few cross-community provisions of the Agreement to spare the blushes of a sectarian party."
Mr Trimble said that, with the General Election and local government elections approaching, unionism could not afford to have five years of the DUP.
"To an extent not grasped here, the DUP, in a House of Commons completely dominated by Labour MPs, are held in scarcely concealed contempt," he said.
"Five years of their sourness will do unaccountable damage to the Union.
"Unionism cannot afford a representation that will make Gerry Adams appear good before the court of English public opinion."
Mr Trimble also said unionists should not be content with direct rule ministers running Northern Ireland's government departments.
He said the introduction of water charges as well as proposed education and local government reforms in the province were all evidence of just how bad direct rule was for Northern Ireland.
Elsewhere at the conference, South Antrim MP David Burnside walked out in protest at some criticism of him from the conference platform.
BBC Northern Ireland political editor Mark Devenport said : "With no Jeffrey Donaldson waiting in the wings, this appeared in general a far more united gathering than the Ulster Unionists have experienced in recent years".