The Tory leadership tonight conceded defeat over their plans to hold a referendum on the Lisbon Treaty.
Shadow foreign secretary William Hague said it was "no longer possible" to put the treaty to a popular vote after Czech President Vaclav Klaus removed the last obstacle to full ratification.
"Now that the treaty is going to become European law and is going to enter into force, that means a referendum can no longer prevent the creation of the president of the European Council, the loss of British national vetoes," he said.
"These things will already have happened and a referendum cannot unwind them or prevent them."
Mr Hague said party leader David Cameron - in a speech to be delivered at 4pm tomorrow - would set out "in detail how we will now go forward in European matters".
He denied that the party had broken any promises by dropping the referendum pledge.
"A British referendum until this very day would have meant that the Lisbon Treaty wouldn't enter into force if people voted no. The position of president of the European Council, the foreign minister of Europe, would never have been implemented," he said.
"We were very clear that our promise applied to those circumstances. After today, those things will come into force and a referendum can't change them, it can't unwind them, it can't prevent those things being created.
"That is why we are now in new circumstances and David Cameron will set out very clearly tomorrow how we now intend to proceed."
Earlier, the deeply Eurosceptic Mr Klaus announced he had signed the treaty after the Czech Constitutional Court finally rejected an attempt to block its implementation.
The move threatens to throw the Tories into a renewed round of turmoil over Europe after years of relative peace.
The Conservative leadership has been signalling for some weeks that it would not be able to go ahead with a referendum if the treaty was ratified by the time a Tory government came to power.
However for Tory Eurosceptics it has become an article of faith after Mr Cameron gave a "cast iron guarantee" two years ago that he would give the British people a chance to vote on the treaty.
He is now expected to announce a general election manifesto commitment to "repatriate" powers from Brussels, including control of social and employment policies - a long-standing Tory aim.