"I always want to keep alive the prospect for a referendum as long as possible. That's why I've always hoped that President Klaus wouldn't sign the treaty," Conservative leader David Cameron said in response to a reporter's question.
"But it looks as if times are changing and we will address that just as soon as we need to," he said.
The Conservatives, favourites to win a national election due by next June, have promised to hold a British referendum on the Lisbon Treaty if it is not in force across the 27-nation EU if and when they take power.
The Conservatives strongly oppose the treaty, designed to smooth decision-making in Europe, seeing it as a step towards a federal Europe.
If the treaty were in force, Cameron has said he would not let matters rest there, but has refused to say what that means.
One of the last obstacles to the treaty taking effect was removed last week when EU leaders agreed to Klaus's demand to be given an opt-out from the EU human rights charter.
The Czech Republic is the only EU member country not to have ratified the document. The last hurdle to Klaus signing the treaty appears to be a ruling by the Czech constitutional court, expected today, on a challenge to the treaty.
If the court clears the way for Klaus to sign the treaty, as expected, the British Conservatives are likely to say soon afterwards what their policy will be.
"What I've said is we want to hold that referendum on the Lisbon Treaty, but clearly it seems we are now getting close to a point where that treaty is no longer going to be a treaty, but will be part of European law," Cameron said.
"We've always said, as and when that happens, we will address immediately what we will do to ensure that we are good to our promise of not letting matters rest there," he said. Instead of a referendum, the Conservatives are expected to seek the repatriation of some powers, such as social policy, from Brussels to London.
Labour Prime Minister Gordon Brown accused Cameron on Monday of demanding withdrawal from EU social and employment legislation when the challenge was to secure economic growth and a climate change agreement.
"Is it going to be the best use of British influence to fight yesterday's battles the minute that the European Union has moved on from them?" Brown said in parliament.
(EurActiv with Reuters.)
