Tony Blair has sparked a major political row by effectively ruling out a referendum on changing Britain's voting system before the next general election.

As the long-awaited Jenkins report called for a move towards PR, the prime minister "warmly welcomed" the proposals, but made it plain there would be no immediate changes.

His official spokesman said a referendum before the next poll - which was a Labour manifesto pledge - was only an "option".

And Mr Blair tried to link the issue with the more far-reaching plans to reform the House of Lords.

The decision leaves Liberal Democrat leader Paddy Ashdown swinging in the wind.

He has staked his leadership credibility on winning a referendum before the next election and one senior party member said Mr Blair's comments "are not good news for us".

Mr Ashdown claimed the report marked "an historic step forward in building a modern democracy for Britain."

Sink without trace

Meanwhile, Labour's First Past the Post group laid down its marker by insisting more than 100 MPs were "implacably opposed" to the Jenkins proposals and claiming they would "sink without trace".

Prescott leads no change camp

The cabinet is also split down the middle on the issue with Foreign Secretary Robin Cook leading the group in favour of change and Deputy Prime Minister John Prescott heading the "no change" camp.

The Tories also promised to fight any changes, with Chairman Michael Ancram opposing a referendum and declaring: "I would like to see this die now."

Mr Blair outlined the report and his reaction during a 20 minute debate in the cabinet on Thursday morning but he also insisted that ministers would not be allowed to campaign for one side or the other during a referendum campaign.

And it is certain the issue will stir up a serious internal debate within both the Labour and Liberal Democrat parties.

It is being claimed by some in Westminster that a deal has been done between Mr Blair and Mr Ashdown in a bid to shore up the Liberal Democrat leader's position while averting a full-scale civil war on the Labour benches.

Commons debate

Mr Blair's comments were the most positive yet about the possibility of change, describing the report as a "well-argued and powerful case" for change.

There will now be a full Commons debate next week and a long public consultation on the report.

It is thought Mr Blair might even go further at some point and hand Mr Ashdown a personal boost by declaring his personal support for change.

But he will rule out any referendum before the next election in a bid to damp down the row in his own party.

The Jenkins report strengthens his arm by saying the changes could not be in place before the election after next.

And, by attempting to tie the plans in with wider constitutional reform, the prime minister is introducing another excuse for delay.

Ashdown welcomed report

The Liberal Democrats welcomed the report and, at a packed meeting of their MPs in Westminster on the eve of its publication, gave Paddy Ashdown a rousing reception for having brought them this far.

Serious problem

But he will have real problems selling the package to his party if Labour refuses him the early referendum.

One senior party member said Mr Ashdown had almost unprecedented support from his MPs who had seen one of their key political demands coming close to realisation under his leadership.

But he was depressed by the signals coming from Downing Street and made it plain that could land his leader with a serious problem.

Even Lord Jenkins was forced to admit that the issue of electoral reform was not a "vast, surging issue" with the electorate.

Many also fear the move could turn people off voting. There are already different systems of PR for the Scottish, Welsh and Northern Ireland assemblies and for the European parliament.

They claim that to add another system would risk alienating voters and threaten even lower turnouts in general elections.