aTHE Prime Minister last night urged the Northern Ireland parties to take a gamble and back the compromise on weapons decommissioning he brokered in Belfast.

Tony Blair urged the Ulster Unionists not to ''throw away the best chance for peace we will have this generation'' and accept a ''failsafe'' mechanism designed to trigger an IRA weapons handover ''within days''.

He also offered them the prospect of a review of the controversial prisoner release programme, which has so far seen more than 250 terrorists freed before completing their sentences. He indicated that the programme could be suspended if republicans backtracked on their promises and the peace process broke down.

But his hopes of winning Unionist support faced a fresh hurdle last night after the independent Parades Commission announced it had banned next week's Orange Order march down the Catholic Ormeau Road in Belfast. The decision threatens to harden Unionist opinion against the London-Dublin plan, which is already under fire amid claims that it concedes too much to the republicans.

Mr Blair said the Government was on standby to rush emergency legislation through the Commons which would implement the new system for triggering paramilitary arms decommissioning and launch the new Northern Ireland assembly on July 15.

In an attempt to woo the Unionists, Mr Blair made clear that IRA failure to hand over arms would mean the immediate exclusion of Sinn Fein from the province's executive.

The blame for default would be clear, he said. The other parties would be free to form a new executive without the republicans.

To reinforce the tough message, his officials said that if the process collapsed the Government would be prepared to review other aspects of the Good Friday agreement, including prisoner releases. ''We will review prisoner releases should we get to that situation,'' the Prime Minister's official spokesman said.

Unionist leader David Trimble made clear in the Commons his unease at the leap of faith Mr Blair is asking him to make. He warned the deal amounted to giving republicans a veto on the peace process. He told Mr Blair: ''You are asking us to sacrifice the democratic principle to expediency and you are asking us to take a gamble on an ineffective and unfair safety net.'' Later Mr Trimble had a seven or eight minute phone conversation with US President Bill Clinton.

The deal worked out by Mr Blair and his Irish counterpart Bertie Ahern would give Canadian General John de Chastelain responsibility for certifying if paramilitary decommissioning was genuinely under way. It sets out a specific timetable of events, with the decommissioning process beginning ''days'' after the executive takes power, and the first handover of arms by the IRA later this summer and completed by May next year.

A Bill would be passed in the next 10 days allowing for the executive to be abolished if at any stage the IRA was certified by General De Chastelain, the chairman of the international decommissioning body, to be failing in its obligations.

''After 30 years of bloodshed, grief-stricken families, terror-torn communities, is it not worth waiting 30 days to see if the undertakings made here are fulfilled?'' Mr Blair said. ''If we don't put this agreement to the test and find out - literally within days - whether it (IRA decommissioning) is going to happen, we will never know.'' He said that it went further than last Easter's Hillsborough declaration, which only called for a ''token'' start to IRA decommissioning.

Privately officials conceded that the likelihood of an executive being set up without Sinn Fein was unlikely. The Unionists argue that unless the nationalist SDLP party led by John Hume agreed to back the formation of an executive without Sinn Fein, the IRA would be given an effective veto on devolution in Northern Ireland.

Sinn Fein president Gerry Adams claimed that the Government's plans to wind up the executive if the IRA defaulted on decommissioning breached the Good Friday agreement.

''Under the terms of the Good Friday agreement this is not possible,'' he said. ''There is no question of the British Government introducing legislation to expel Sinn Fein, Mr Blair knows this would be a breach of the Good Friday agreement.''

Mr Blair's hopes of winning Unionist support suffered a setback with the decision to bar Orangemen from their traditional march route through the Catholic Ormeau Road after they were allowed to parade last year. The Orange Order had hoped for a reprieve after successfully keeping the peace in Drumcree following the ban on its march down the nationalist Garvaghy Road. But the Commission said in its determination that failure of the Orangemen to engage fully in local discussion with the residents' group was behind its decision.

George Patton of the Orange Order said he was ''very disappointed and frustrated'' at the decision not to allow the Ballynafeigh Orangemen to march.

Gerard Rice, of the nationalist Lower Ormeau Concerned Community, said: ''The community is absolutely and totally relieved.''

Drumcree itself remained low key as 1000 people were contained ed on the Orange side. Some youths managed to break through security force lines. Fireworks were thrown as a helicopter with a searchlight shone down on the scene.