David Cameron, the Conservative leader. Photograph: Murdo MacLeod
William Hague tonight confirmed that the Tories will not hold a referendum on the Lisbon treaty, which was signed by the Czech president today.
Václav Klaus completed the EU agreement's progress by formally signing the treaty – bringing the union's new "rulebook" into force at the end of a tortuous ratification process.
David Cameron had urged Klaus to keep on blocking ratification, but found himself isolated when the Czech leader put aside his own dislike of the treaty and his objections to a court ruling and signed.
In a pooled interview to TV news channels, the shadow foreign secretary said theConservatives would not now hold a referendum on the treaty if they won the next election.
Hague said it was "no longer possible" for an incoming Conservative government to hold such a plebiscite. "This is a bad day for democracy," he said, according to Sky News.
"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," the shadow foreign secretary added. "These things will already have happened and a referendum cannot unwind them or prevent them."
Hague said that Cameron would tomorrow set out "in detail how we will now go forward in European matters".
He denied that the Conservative party had broken any promises by dropping its 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."
Cameron faces a backlash from some Tory Eurosceptics over his change in policy, which the Guardian predicted last month.
Today he said he was "very disappointed" by the Czech court decision. His speech tomorrow will come on a day when Westminster is preoccupied with the publication of the report from the committee on standards in public life on MPs' expenses.
Many Tory Eurosceptics are backing Cameron, but some of them claim that ditching the referendum commitment would amount to a breach of trust with the electorate.
All EU countries apart from the Czech Republic had already ratified the Lisbon treaty before today and the only person stopping it coming into force was the Eurosceptic Czech president, who said he would not sign while the treaty was still being challenged in the Prague courts.
But this morning the Czech constitutional court dismissed objections lodged by a group of Czech senators who claimed the treaty launches a European superstate and is incompatible with the country's constitution.
In an interview on LBC before Klaus signed the treaty, Cameron said that he would be entitled to drop his referendum pledge after ratification because the treaty would cease to exist and instead be part of European law.
"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, we've challenged the prime minister about his broken promise in the Commons, we've tried to persuade other European countries not to sign the treaty, because we think the British people should be allowed a referendum," Cameron said.
"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."
Cameron added: "If this treaty becomes law, it becomes law along with all the other treaties that have been passed into European law and 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."
Cameron's announcement tomorrow is expected to include a reaffirmation of the Tories' plan to change the law so that any future government would be legally required to hold a referendum before transferring powers to the EU. Hague originally announced this policy in 2007.
Although many Tory Eurosceptics appear to support Cameron's new approach, some of them want him to press ahead with a referendum on Lisbon regardless of what happens in Prague.
Roger Helmer, the Tory MEP, told the BBC's World at One: "[Cameron] is a man of his word ... He gave a cast-iron guarantee to the British people that he would have a referendum if and when he became prime minister and I believe that that is a commitment he will need to keep to."
Helmer denied that a referendum would be pointless. He said it would "strengthen the negotiating position of the British government" in talks with Brussels about repatriating powers to the UK.
The Eurosceptic MP Richard Shepherd also urged Cameron not to abandon his plans for a referendum. "It's a matter of principle, honour and trust," Shepherd told the London Evening Standard.
The foreign secretary, David Miliband, said: "So much for David Cameron's cast-iron guarantee to hold a referendum on the Lisbon treaty. 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. I gather that he is about to give another cast-iron guarantee that he can't and won't honour, this time on the repatriation of key competencies from the EU that affect British people, such as social policy. In practice the EU social chapter has delivered important rights for British workers such as paid leave, anti-discrimination laws and maternity leave.
"The fact is you can't simply opt out of treaty obligations because to do so you need the agreement of the 26 other member states. The concessions David Cameron would have to offer would be costly and weaken Britain considerably."
The treaty was conceived after the EU constitution was thrown out by French and Dutch voters in 2005. Rejected by Irish voters in June 2008, it got overwhelming support in a second referendum in Ireland in October, clearing the way for today's final ratification. It could be in force as soon as next month.
The treaty streamlines the workings of the EU, and creates two new positions: president, and high representative for foreign affairs. Tony Blair is a possible candidate for the first job, Miliband for the second.