Conservative leader David Cameron will be forced to announce a new policy on Europe on Wednesday, after the Lisbon Treaty cleared its final hurdle.
The Conservatives will not hold a referendum on the EU treaty if they win a general election, shadow foreign secretary William Hague has announced.
The treaty is set to become law across Europe on 1 December after Czech President Vaclav Klaus became the last EU leader to sign it.
Mr Hague said ratification of the treaty by the Czechs - the last EU country to do so - meant the Tory campaign for a referendum "ends today".
Mr Cameron has said he was disappointed by President Klaus's decision.
He said Tory leader David Cameron would set out details of a new policy on Europe on Wednesday.
The Tory leader is under pressure to hold a post-ratification referendum if he wins the next election.
Mr Cameron has been under pressure to hold a post-ratification referendum.
Eurosceptics have accused Mr Cameron of reneging on a promise, made in a 2007 article for The Sun newspaper, in which he said: "Today, I will give this cast iron guarantee: If I become PM a Conservative government will hold a referendum on any EU treaty that emerges from these negotiations."
'New situation'
Tackled about this pledge at a press conference, Mr Cameron said: "I think it is only fair that people should be given a referendum."
But he added: "It looks like this treaty is no longer going to be a treaty, it looks like it is going to become part of European law and that is going to create a new situation."
He said the party would be "responding in full before the end of the week" to the "new situation". Tory sources said an announcement would be be made on Wednesday.
Mr Cameron said the public had been denied a referendum on the Lisbon Treaty by Labour, which he said had promised one in its manifesto.
Labour claims the Lisbon Treaty is not the same as the now defunct constitutional treaty, which it had promised a vote on.
Czech foreign minister Stefan Fule is expected to deliver the signed Lisbon treaty - known as "depositing the articles" - in Rome later on Tuesday.
In a statement, Prime Minister Gordon Brown said: "President Klaus's decision marks an important and historic step for all of Europe.
"Today is a day when Europe looks forward, when it sets aside years of debate on its institutions and moves to take strong and collective action on the issues that matter most to European citizens: security, climate change, jobs and growth."
'Not being honest'
Foreign Secretary David Miliband accused Mr Cameron of taking a "false and dangerous" stance on the treaty that risked Britain's national interests.
"As British foreign secretary, I would welcome Mr Cameron's U-turn because this treaty is good for Britain and for the British people. But he is still not being honest with people."
Conservative sources have confirmed to the BBC that Mr Cameron's new policy on Europe will be unveiled on Wednesday.
They said they were not deliberately timing the announcement to clash with the publication of Sir Christopher Kelly's report on MPs' expenses, insisting the party had always promised to unveil the policy as soon as they could and did not wish to be accused of "dithering".
It has been suggested that the Conservatives could seek to renegotiate some of the powers contained in the treaty or demand some powers back from Europe in social affairs, employment or justice if they win the next election, as well as vowing a referendum on any future European treaties.
But this is unlikely to satisfy those demanding a referendum, who are calling on Mr Cameron to honour what they believe was a "cast iron" promise to hold one no matter what.
Leading Conservative Eurosceptic Bill Cash told The Daily Telegraph: "We need a full referendum on Lisbon as we were promised. No ifs no buts.
"This is about the government of the UK operating in line with the democratic wishes of the country."
Last major obstacle
Conservative MEP Roger Helmer said he believed Mr Cameron was "a man of his word" as he had kept his pledge to remove the Tories from the centre right European People's Party in Brussels and Strasbourg.
"He gave a cast iron guarantee to the British people that, if and when he became prime minister, we would have a referendum and I believe that is a commitment he will need to keep to," he told BBC Radio 4's the World at One.
David Cameron must come clean on where he now stands Ed Davey, Lib Dem foreign affairs spokesman
But Tory MP Philip Davies, of the Better Off Out group, said there was little point in holding a referendum on a treaty that had already been ratified and instead there should be a referendum on whether Britain should remain in the EU.
Lord Pearson, for the UK Independence Party, which campaigns for Britain's exit from the EU, warned Mr Cameron that if he did not pledge to hold a referendum the Conservatives would lose many votes to UKIP at the next election.
"If they don't make sense on this issue we will cost them at least 50 seats," the peer told the BBC News channel.
Liberal Democrat foreign affairs spokesman Edward Davey said that if Mr Cameron cannot make his position on Europe clear "he is not fit for government".
He added: "David Cameron will be terrified of betraying the Eurosceptic wing of his party, but he must come clean on where he now stands."
Sources say there is likely to be an EU summit next week to discuss who will fill the posts of President of the European Council and High Representative for Foreign Affairs, which will be created when the treaty comes into force.
Former UK Prime Minister Tony Blair is widely seen as a candidate for president.
Despite his insistence that he is "not available" to take the job, UK Foreign Secretary David Miliband is being considered as a possible High Representative - in effect Europe's first foreign minister, sources have told the BBC.
