Ulster Unionists will hold an extraordinary general meeting of their ruling council on June 23rd to elect a new leader, the party's executive decided on Saturday.
Until then the party's sole MP, Lady Sylvia Hermon, party president Lord Rogan and Assembly member Sir Reg Empey will fulfil a leadership role. The three hold their first meeting early this morning.
A UUP delegation is due to meet the new Northern Secretary Peter Hain and others in the short term, and the executive wanted established figures to represent them.
The 3½-hour meeting was "emotionally charged", according to one delegate, "but not rancorous".
Delegates, many still said to be in shock following last week's hammering at the polls, heard the text of the short resignation letter by David Trimble before the former leader left the meeting.
One source said Mr Trimble received a "standing ovation" and "prolonged applause".
A motion from the party's 24-member Assembly group formed the basis for a compromise measure, adopted by the executive, which opted for a leadership election next month rather than immediately or delaying it until later this year or until early 2006. Lord Kilclooney, the former Strangford MP John Taylor, had offered to act as interim leader until the next agm of the ruling Ulster Unionist Council next spring.
Others, notably Lady Hermon, were pressing relentlessly for a clean break with the previous leadership and an early election.
Many Ulster Unionists believe a deal could be struck by Sinn Féin and the DUP, now firmly in leadership positions in their respective political traditions. Any deal to restore power-sharing at Stormont could involve fresh Assembly elections, and delegates want their new leader to be as well-established as possible.
The ruling Ulster Unionist Council will now be summoned for the leadership contest in just over five weeks. Following changes to the party constitution and the decision of the Orange Order not to maintain formal links with the party, about 600 delegates will vote on Mr Trimble's replacement.
Key unionists figures have yet to confirm their interest in the job, but the defeated South Antrim MP David Burnside has made clear his opposition to a Hermon-led party.
"If the party wants to go off on some sort of softy, wishy-washy, liberal sort of route, they'll have a lot of other people who are still in the party stepping aside from it," he told the BBC on Saturday. "We don't need to go that route."
Opinion is divided between those, like Mr Burnside, who advocate a tougher line on the Belfast Agreement and others, including Lady Hermon, who want clearer distinctions between the party and the DUP, including a fresh restatement of the party's liberal and centrist credentials.
"Unless we do that, we are dead in the water," she said.
Meanwhile Sinn Féin president Gerry Adams has pressed for "the peace process to be put back on track".
Mr Adams confirmed his party had held meetings with ministers and government officials, adding that there was now a "real opportunity" to achieve progress based on the Belfast Agreement, which he described as being in "cold storage". "We want to see the Good Friday agreement implemented in full," he said.