Tory leader David Cameron has abandoned plans for his promised European referendum after Czech president Vaclav Klaus signed the Lisbon Treaty.

Shadow foreign secretary William Hague said it was "no longer possible" to put the treaty to a popular vote of the British people.

"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."

Earlier, Mr Klaus completed the process by formally signing the treaty - ushering in the EU's new rule book after a long ratification process. Mr Cameron, who had urged Mr Klaus to keep on blocking ratification, found himself isolated when the Czech leader put aside his misgivings and signed the document.

Eurosceptic Tory backbencher Bill Cash said he had written to Mr Cameron urging him to change his mind and honour the referendum pledge. But Mr Hague said the party leader would set out "in detail how we will now go forward in European matters" during a speech on Wednesday.

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."