The former US president George Bush has made a direct plea to David Cameron to support the Northern Ireland peace process, amid widespread concern in the US about the Tories' new electoral pact with the Ulster Unionists.

In his most active intervention since leaving the White House, Bush took the rare step of calling the Conservative leader to ask him to use his influence to press his unionist partners to endorse the final stages of the 15-year search for a settlement.

The intervention by Bush, in a telephone call last Friday, appeared to have failed last night when the Ulster Unionist party confirmed that it would vote against the devolution of policing and criminal justice powers to Belfast.

The unanimous decision by the party executive means that the once mighty UUP, which governed Northern Ireland until direct rule was imposed in 1972, will be the only member of the four-party power sharing executive that will vote no today. The Democratic Unionists and Sinn Fein, which brokered the agreement on policing last month, will join the SDLP in voting for the deal.

The decision by the UUP to oppose what London describes as the final piece in the jigsaw of the 1998 Good Friday agreement will come as a blow to Cameron and will cause alarm in the US. The Tory leader supports the devolution of policing powers to Belfast but appears to have failed to persuade his political allies in Northern Ireland to follow his lead.

Amid alarm in the US at the prospect of a UUP no vote, Bush telephoned Cameron last Friday to ask him to plead with the UUP leader, Sir Reg Empey, to endorse the deal. While the UUP does not have enough votes to scupper the deal, political leaders in the US fear a no vote from the UUP could undermine support for the settlement within the DUP and among the wider unionist community.

The Guardian understands that the White House is so concerned that the US economic envoy to Northern Ireland, Declan Kelly, persuaded Bush to intervene. The former president, who took a close interest in the peace process during his years in the White House, telephoned Cameron to ask him to use his influence to persuade Empey to vote for the deal.

"There was a feeling that a conservative to conservative conversation was the right way to go about this," said one source familiar with the transatlantic negotiations. "This conversation was borne out of the concern that Empey is holding out." Another source familiar with the contact said: "This is the most active thing George W Bush has done in his post-presidency period. He has been incredibly restrained and diplomatic since leaving the White House. He has maintained radio silence."

One source familiar with thinking on Northern Ireland on both sides of the Atlantic added: "The fact that George W Bush has decided to intervene is really significant. He was interested in the peace process as president and appointed an envoy. It is a general sign of how concerned people are in the US about what David Cameron is up to."

A Tory source said: "George Bush did have a conversation with David Cameron. It was a positive conversation. David underlined his commitment to the agreement and said we are doing all we can to support it. But he said that we cannot force Sir Reg to vote for it. George Bush thanked David and said, 'I can see you are engaged.'"

Democrats and Republicans will be bitterly disappointed with last night's decision by the UUP. As one of the main architects of the Good Friday agreement in its former days as the Northern Ireland's largest party, the party traditionally reflects middle ground unionism.

But Empey, who is minister for employment and learning in the executive, believes it is foolish to devolve the final set of powers while the coalition is struggling to function. A meeting between the UUP and the Sinn Féin deputy first minster, Martin McGuinness, broke up after just three minutes last night.The White House has been watching developments with care. Hillary Clinton, the US secretary of state, yesterday pleaded with Empey in a phone call to support the deal.

A group of US Congress members wrote to Cameron last month to issue a stark warning that dissident terrorists will be "emboldened" to intensify their attacks if he fails to persuade the Tories' partners to endorse the final stage of the peace process. Cameron has faced pressure in recent weeks over his decision to form an electoral pact with the UUP, which could provide him with crucial support if the general election leads to a hung parliament. The pact has prompted fears in Washington that he may abandon the even-handed approach to Northern Ireland that was adopted by John Major.