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Conservative leader David Cameron is to unveil his new policy on Europe, after admitting a referendum on the Lisbon Treaty will no longer be possible.
Mr Cameron is coming under intense pressure from eurosceptic MPs and activists to offer a vote of some sort on Britain's relations with Europe.
But it is thought more likely that he will use a speech in London on Wednesday to promise a manifesto commitment to seek to repatriate powers over social and employment policies from Brussels to Westminster.
This would require the agreement of all 27 EU states, and it is thought possible that Mr Cameron will use the threat of a referendum on future treaties - such as those implementing the accession of new members such as Croatia - in order to secure their acquiescence.
When the Lisbon Treaty was being negotiated in 2007, Mr Cameron gave what he said was a "cast-iron guarantee" of a referendum on any document emerging from the process. He repeatedly mocked Prime Minister Gordon Brown for backing away from his promise of a public vote on the European Constitution - which Labour insists was a different document from Lisbon, but Tories claim was effectively the same.
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But after Czech President Vaclav Klaus finally completed the ratification of the Treaty on Tuesday, shadow foreign secretary William Hague acknowledged that plans for a referendum on Lisbon were being dumped.
"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," said Mr Hague. "These things will already have happened and a referendum cannot unwind them or prevent them."
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, though Mr Cameron has promised he would not "let matters rest".
Eurosceptics within the party were on Tuesday night putting pressure on him to allow voters to express their views on what they see as a decades-long process of closer European integration.
