Tony Blair's endeavour to achieve a breakthrough in the Northern Ireland peace process came to a shuddering halt yesterday when he conceded that the IRA had not met his demands for a promise to end paramilitarism.

The Prime Minister reluctantly announced the postponement of this month's Belfast Assembly elections to the autumn, a move criticised by a spectrum of opinion which, unusually, spanned the Irish government, Sinn Fein and the Rev Ian Paisley.

Only the Ulster Unionist leader, David Trimble, approved of the postponement, for which he has been pressing. Mr Blair was attacked for favouritism towards the Unionists.

While the republicans and Mr Paisley accused the Prime Minister of denying democracy by putting off the poll, he defended his action on pragmatic grounds. "There would be little point in having an election if there was no prospect of getting a government out of it."

The Government published a lengthy document that it had drawn up witDublin, holding out the prospect of major reductions in security bases and troop levels. But this and a series of other initiatives are clearly contingent on an IRA declaration of pacific intent, and Mr Blair accused the republicans of failing to deliver it. Among his criticisms was the IRA's failure to honour paragraph 13 of the document, which deals with paramilitary activity.

Mr Blair objected to Sinn Fein president Gerry Adams' "point-blank refusal" expressly to rule out activities including training, targeting, intelligence gathering, acquisition, punishment attacks and involvement in riots.

Mr Blair's calculation was that without such specific assurances, David Trimble would not attempt to persuade his party to go back into government with Sinn Fein.

This absence of any understanding between the two most important parties meant that a new power-sharing coalition was unlikely, while many also believed an election in these circumstances would damage the Trimble showing and enhance a Paisley protest vote.

The Dublin government made it plain yesterday that it disagreed with such an approach, partly because some Irish government elements do not share Mr Blair's distaste for Mr Paisley's party.

The Prime Minister made a point of saying that Paisleyite involvement in power-sharing was "pie in the sky".

In a characteristically forthright rebuke, Mr Paisley said: "It is an appalling thing that the IRA/Sinn Fein have been talking to the Government when the Unionist population have been treated like lepers."

The Government can console itself that it extracted much valuable language from the IRA and Sinn Fein over recent weeks. But it failed to come up with the vital last missing part of the jigsaw.

Since the summer marching season is regarded as an unpromising time for a breakthrough, the Government will now make another attempt to clinch a deal on elections in the early autumn.

By Mr Blair's own logic, however, there would be little point in holding elections at that stage in the absence of increased IRA clarity. Any prospect of progress therefore seems to rest on the IRA reversing its present stance and agreeing to be much more specific.

Mr Adams said: "When I made my remarks on Sunday and Mr Blair queried my answer on the issues of IRA activity, I gave him his answer and I gave him precisely the answer he asked for at that time. He's now looking for a different answer. So rather than today moving the process forward, it has in many ways compounded the difficulties."

Bertie Ahern, the Irish Prime Minister, said he disagreed with the Government's decision to delay elections. "Ultimately, I believe that yet another postponement causes more problems for the process than it solves," he said.