Conservative leader David Cameron has denied his party is in turmoil after he was forced to abandon a promised referendum on the Lisbon Treaty.
The ratification of the treaty by the Czech Republic on Tuesday scuppered the UK opposition party's plans for a vote on it if they win power next year.
But Mr Cameron is still under pressure from Eurosceptics in his own party to offer the public a say on Europe.
Gordon Brown said Mr Cameron had "devalued" his own policy on Europe.
During prime minister's questions, the Tory leader was repeatedly taunted by Labour MPs over comments he made in 2007 promising a "cast-iron" commitment to a referendum on the Lisbon Treaty.
Former Home Secretary David Blunkett said that Mr Cameron's commitment had now "turned to plywood".
Mr Cameron, who did not address the subject of Europe at PMQs, is due to announce his new policy on Europe at 1600 GMT.
Earlier, he denied he had done a U-turn and betrayed his supporters on Europe. He told GMTV: "What has happened is that the politicians who run this country have given away a lot of power to Brussels without asking people first.
"I am going to make sure that that never, ever happens again. That's what today is going to be about."
'Cast iron guarantee'
Asked if the Conservative Party was once again in turmoil over Europe, he said: "Not at all. The party actually wants us to have a fresh approach in Europe but above all the vote that we need is a vote to get rid of a government that has completely let people down."
The fact is you can't simply opt out of treaty obligations because to do so you need the agreement of the 26 other member states
Mr Cameron could promise referendums on future European treaties and to fight for the repatriation of some powers from Brussels, in his statement.
But that is unlikely to satisfy the Eurosceptic wing of his party, which has accused him of reneging on a "cast iron" guarantee to offer the public a referendum on the treaty if he becomes prime minister.
Eurosceptic Conservative MP Bill Cash said he had written to Mr Cameron urging him to "reconsider" his decision not to hold a referendum, saying the Tory leader had been "badly advised".
Mr Cameron - who opinion polls suggest is on course to win the general election, widely expected in May - was forced to abandon his pledge to hold a referendum on Tuesday after Czech President Vaclav Klaus signed the Lisbon Treaty.
He had been urging Mr Klaus to delay signing the treaty until after a British general election so that a referendum could be held in the UK.
'Good for Britain'
But he said the Czech ratification, which means the treaty will pass into law across Europe on 1 December, created a "new situation".
Shadow foreign secretary William Hague described Tuesday's events as a "bad day for democracy".
But he said it meant the Conservatives' long campaign against the treaty, which will create a President of the European Council and High Representative for Foreign Affairs, effectively a foreign minister, had to come to an end.
It is no longer possible to trust the Tory party or David Cameron when they make promises about Europe
He blamed Labour for reneging on its promise to hold a referendum, saying: "We have campaigned for that referendum for many years, we believe passionately that there should have been a referendum so that the British people could be consulted."
Labour argues that the Lisbon Treaty is "good for Britain" and different to the ill-fated constitutional treaty on which it had promised a referendum.
Foreign Secretary David Miliband said David Cameron was "still not being honest with people" about Europe, amid speculation the Tory leader will promise to demand powers back from Europe in areas such as social affairs, employment and justice.
'False and dangerous'
Mr Miliband said: "The fact is you can't simply opt out of treaty obligations because to do so you need the agreement of the 26 other member states.
"David Cameron's position on Europe is false and dangerous."
Mr Cameron will be hoping to avoid a repeat of the civil war over Europe which marred the leadership of many of his predecessors as Conservative leader.
Conservative MEP Daniel Hannan, who has called for Britain to leave the EU, said: "Alright, a referendum on the Lisbon Treaty might no longer be the most logical option: it's hardly for us to tell the Belgians or the Slovenes what institutions they should work under.
"But a referendum on European integration - ideally on the broad repatriation of powers - is essential."
Former shadow home secretary David Davis, who was defeated by Mr Cameron for the Conservative Party leadership in 2005, has called for a referendum on Europe within three months of a general election.
Writing in the Daily Mail, Mr Davis said the vote was needed to give a negotiating mandate for a future Tory government to take to the EU.
The referendum question should incorporate aims such as "recovering control over our criminal justice, asylum and immigration policies" and a "robust opt-out of the European Charter of Fundamental Rights", he wrote.
The UK Independence Party, which campaigns for Britain's exit from the EU and came second to the Conservatives at this year's European elections, is hoping to win votes off the party at the next election.
UKIP leader Nigel Farage said: "Mr Hague says it is 'no longer possible' to have a referendum. Well, to me and millions of others it is apparent that it is no longer possible to trust the Tory party or David Cameron when they make promises about Europe."
Liberal Democrat foreign affairs spokesman Ed Davey accused Mr Cameron of being "terrified of betraying the Eurosceptic wing of his party", adding he "must come clean on where he now stands".
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