John Prescott, the Deputy Prime Minister, Gordon Brown, the Chancellor and Jack Straw, the Home Secretary, are to join forces in an attempt to prevent Mr Blair calling a referendum on electoral reform before the next general election.
Mr Brown's opposition could hamper Mr Blair's attempts to put on a united front with his Chancellor, after faction-fighting between their respective allies was blamed for the resignations of two senior ministers - Peter Mandelson and Geoffrey Robinson - and Charlie Whelan, Mr Brown's press secretary.
A source close to Mr Brown said: "He doesn't understand why we need to offer the Lib Dems proportional representation [PR]. He believes first- past-the-post is a better, more clear-cut system."
Mr Brown, Mr Prescott and Mr Straw are increasingly confident of blocking a referendum since the departure of Mr Mandelson, who was the Cabinet's strongest supporter of closer Lib-Lab relations.
Sources close to Mr Blair conceded yesterday that an early referendum was "less likely" but insisted the Prime Minister could still call one before the next election if he felt he could win public support for a new voting system for Westminster.
Mr Blair insisted this week that links with the Liberal Democrats would "intensify" but senior ministers claim he is in a clear minority in his own Cabinet. "The balance of forces on this issue has changed considerably," said one.
Mr Ashdown, under fire in his own party for "cosying up" to Mr Blair, is bracing himself for further criticism as the prospect of a referendum recedes.
Although the Liberal Democrat leader's critics will not move against him in the run-up to elections to local authorities, the Scottish and Welsh assemblies and the European Parliament this spring, he will face a showdown over Lib-Lab relations at his party's conference in Harrogate in September.
There could be moves to replace Mr Ashdown with a leader less keen on close links with Mr Blair, such as Charles Kennedy, the agriculture spokesman, or Simon Hughes, the health spokesman.
Last night, Mr Ashdown's allies doubted that his opponents would force a leadership contest and said he would not consider standing down until after the next general election. "He doesn't think there is a successor yet; he is still enjoying things and achieving things," said one.
In a further blow to Mr Ashdown, opposition to electoral reform inside the Labour Party has forced Mr Blair to put off a decision on the issue until autumn next year, further reducing the prospect of a referendum before the next election, expected in 2001.
Supporters of reform had hoped that the Labour conference this October would decide on plans by a commission headed by Lord Jenkins of Hillhead, which recommended a watered-down form of PR called the "alternative vote plus".
The Prime Minister said the Jenkins report made "a well- argued and powerful case" but has put off a decision in the hope of winning over hostile Labour activists and MPs. "The driving force for closer Lib Dem links was not Peter Mandelson; it was Tony Blair," one Blair aide said last night.
A defiant Mr Blair, speaking in Cape Town as he completed a three-day trip to South Africa, declared there would be no turning back from Labour's "third way" policies and rejected a return to old Labour policies.
The last day of his visit was marred by clashes between Muslim protesters and police at Cape Town's 17th-century Dutch castle where three people were hurt when police opened fire with shotguns, tear gas and stun grenades.
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