David Cameron was put firmly on the spot today when a Czech constitutional court removed the last possible legal barrier to the Lisbon treaty. The judges' decision to give the green light to the controversial EU accord means that it should come into force by the end of the year after a long-awaited flick of the pen from Vaclav Klaus.

It dashes the hopes of the Conservatives, who had relied on the Czech President to delay its ratification until after the next general election in the UK.

The court in Brno had spent the past week considering a petition by 17 Eurosceptic Czech senators but it ruled this morning that the treaty could proceed. The treaty did not, said the chief judge Pavel Rychetsky, contravene the letter or the principles of the country's constitution.

Mr Klaus, who fervently opposes the treaty, has promised to sign the ratification documents if the court agreed that the Lisbon accord did not impinge upon Czech sovereignty and will now come under immense pressure to do so.

The other 26 European Union countries have all ratified the treaty and the Czechs last week won an opt-out guaranteeing that they would not be subject to property compensation claims from relatives of ethnic Germans expelled after the War.

The court's decision came as it emerged that Mr Cameron was planning to drop his party's pledge to hold a referendum on the treaty. The Times reported today that Mr Cameron is preparing to set out a fresh stance on Europe that will involve promising in his election manifesto to "repatriate" certain key powers from Brussels.

Chris Bryant, the Europe Minister, said it was now clear that Mr Cameron would be forced to abandon his "cast iron guarantee" that a Conservative government would hold a referendum on the treaty.

"Now he is clearly saying that there is not going to be a referendum so his cast iron is already rusting pretty badly," he told the BBC.

"I think this is a matter of trust, whether you can really trust David Cameron with the British interest. I think that yet again David Cameron is trying to fudge the issue, he is trying to fib to the British people. Any guarantees that he offers this week will not be worth the press release paper they are written on."

The Liberal Democrat spokesman Ed Davey added: "David Cameron will be terrified of betraying the Eurosceptic wing of his party, but he must come clean on where he now stands.

"He cannot bleat about transparency and then go Awol as soon as there are tough questions to answer. This dithering and evasion shows he is now the heir to Brown more than the heir to Blair. If he can't make his position clear on an issue as crucial as Europe, he is not fit for government."

Mr Cameron himself expressed his disappointment at the court's decision and indicated that the party would set out its new approach "probably later this week".

"We'll have to explain what a Conservative government would do to try and make sure that Britain had her rights protected and defended properly," he said.

Asked on London's LBC Radio whether he had let voters down over the promise of a referendum, he said: "No, I haven't and I won't. I believe we should have a referendum and we've campaigned for it, we've fought for it, we've put it up front and centre at election campaign after election campaign...

"But if the Treaty is signed, if it is implemented, if it is put in place by all 27 countries, then clearly the situation will have changed and we'll have to address that changed situation. It won't be a Treaty any more, it will be part of European law. Now, that looks like that is going to happen, I'm very disappointed about that."

Mr Cameron said he still hoped that Mr Klaus would block ratification. "But I suspect now the time is running out and we will have to say 'Right, OK. a new set of circumstances exists and we'll have to address ourselves to them' and I'll be doing that probably later this week."