David Cameron is being urged by an influential group of United States politicians to persuade the Ulster Unionists with whom he has forged an electoral pact to change their minds and vote tomorrow in support of a crucial step in preventing Northern Ireland’s power-sharing institutions from collapse.

In unusually critical language, a statement from the House of Representatives Friends of Ireland and the Ad Hoc Committee on Irish Affairs calls on the Conservative leader to use his “considerable influence with his election partners in the UUP and convince them to take a risk for peace”.

It adds: “As a supporter of the Hillsborough Agreement he should aggressively be seeking unanimous cross-community support for the landmark accord.”

The Hillsborough Agreement was concluded last month after two weeks of negotiation during which Gordon Brown and his Irish counterpart spent three unsuccessful days trying to broker a deal.

The sticking point has been concessions to the exclusively Protestant Orange Order on the handling of their most controversial parades in return for the devolution of policing and justice powers from London to Belfast.

The Ulster Unionists said on Friday that they would not be voting in favour of the transfer of the powers when it comes before the Northern Ireland Assembly tomorrow because of insufficient progress on other issues and the continuing dysfunctionality of the power-sharing Executive.

But their refusal to vote is being interpreted as a protest at the manner in which the Hillsborough deal was negotiated between the province’s two main parties, the Democratic Unionists and Sinn Fein, excluding other parties.

The Congressmen say in their statement that republican terrorists who in recent weeks have bombed a courthouse, attempted to blow up a police station and murdered a man, will be “emboldened if they sense there is no political unanimity on the way forward”.

Their statement says: “On Tuesday the eyes of the world will be on Belfast and the Ulster Unionist Party. The choice they make will have significant and lasting consequences.

“They can vote yes for a more peaceful and prosperous future. Or they can be on the wrong side of history and vote for the past.”

Mr Cameron is in the process of forging a closer union with the Ulster Unionists, but has not yet been able to announce a full list of joint candidates for the forthcoming election in Northern Ireland’s eighteen constituencies.

The Conservatives accept that the pact is fraught with dangers but believe it will help to bring Northern Ireland politics into the national mainstream.