PRESIDENT Bill Clinton declared himself an Irish Protestant yesterday and appealed to undecided voters in the North to back the peace agreement in Friday's referendum or face a return to violence.

With just 72 hours left to swing wavering Unionists behind the Good Friday deal, Mr Clinton invoked his Ulster roots to launch an emotional appeal on behalf of Northern Ireland's children.

With polls suggesting a significant block of undecided voters hold the key to a decisive ''yes'' vote, Mr Clinton and Prime Minister Tony Blair kept up the pressure in support of the settlement. They said there was no alternative to the settlement.

In London yesterday, Mr Clinton lent his weight to the ''yes'' campaign. ''If I were an Irish Protestant, which I am, living in Northern Ireland instead of the United States, I would be thinking about my daughter's future and her children's future,'' he said.

And he predicted: ''I believe a lot of undecided people will go and vote with their hopes instead of their fears.''

Mr Blair, who is due to make an eve-of-poll visit to Northern Ireland tomorrow, told the same news conference: ''Every generation gets its chances. This is the chance for this generation in Northern Ireland and we have done our best to provide it, but in the end it is their decision.''

But there were signs last night of a further erosion of the ''yes'' vote following last week's prisoner releases, which allowed the IRA's Balcombe Street gang and loyalist murderer Michael Stone to be paraded before the cameras.

It came as hardline Ulster Unionist MP Jeffrey Donaldson dashed hopes that he might switch his ''no'' vote to support for the agreement. A pivotal figure who could have swayed fearful Unionists to back the deal, he said he could not support an agreement that could open the way for Sinn Fein president Gerry Adams to become a minister in a devolved Northern Irish administration.

He added: ''Nor can I support the premature release of terrorist prisoners, some of them guilty of mass murder in Northern Ireland, on both sides, without requiring the terrorist organisations to which they belong to actually deliver peace.''

The British and Irish governments are playing down the possibility of a ''yes'' majority built on mostly Catholic support, with fewer than 50% of Unionists backing the deal.

The prospect of the so-called ''nightmare scenario'', reinforced by a Gallup poll in yesterday's Daily Telegraph, was raised by Ulster Unionist chief whip William Ross, who predicted a tiny majority for the agreement, but with a large number of unionists opposed.

Tonight Ulster Unionist leader David Trimble and SDLP leader John Hume will take an unprecedented step in their bid to bolster the ''yes'' vote.

They will take to the stage at a rock concert in Belfast featuring the Irish band U2. With two-thirds of young voters expected to back the deal, the leaders of the two largest parties have agreed to a joint appearance at the city's Waterfront Hall.

Meanwhile, Liberal Democrat leader Paddy Ashdown will visit the province tomorrow with Alliance leader Lord Alderdice.

He will meet RUC Chief Constable Ronnie Flanagan, RUC officers, a Royal Irish Regiment barracks in Armagh and an integrated school in Belfast.

q The outcome of an application for judicial review on the cases of two Scots Guardsmen jailed for life will also be announced on Friday, the Government confirmed last night.

Junior Ulster Minister Lord Dubs told the Lords that reserve judgment would be given on the plea made for James Fisher, 29, and Mark Wright, 24, who were imprisoned for the shooting of Roman Catholic Peter McBride while they were on an Army foot patrol in Belfast in 1992.

Lord Dubs was speaking in second reading debate on the Police (Northern Ireland) Bill.