The Czech Republic threw the future of the Lisbon treaty in doubt yesterday by demanding an amendment, even though the document has already been accepted by the rest of the EU.

The delay while the details are thrashed out — all member states must agree to changes — could allow David Cameron, the Conservative leader, to fulfil his promise of a British referendum.

Vaclav Klaus, the notoriously unpredictable Czech President, is seeking unspecified guarantees on the Charter of Fundamental Rights — the declaration being incorporated into EU law by the treaty and from which Britain has an opt-out. After the Irish referendum last week, and with Poland about to fall into line, only the Czech leader’s signature is now holding up ratification.

Mr Cameron has declined to spell out what he would do if he came to office after the treaty had been ratified. Eurosceptics in his party want him to negotiate the repatriation from Brussels of powers over employment and social laws. He said yesterday that he wanted to “return to democratically accountable politics those powers the EU should not have” — a course that constitutional lawyers say would be extremely difficult.

Mr Klaus, who regularly compares the EU to the Soviet Union, refused to clarify his demand for a “footnote” to the treaty on the charter, but experts warned it could take months of negotiation with the other 26 member states.

Poland announced that President Kaczynski would sign the treaty in Warsaw tomorrow. Mr Kaczynski had always said that he would give his assent as soon as the Irish voted “yes”.

As anger mounted in EU capitals last night over Mr Klaus’s demand, Bernard Kouchner, the French Foreign Minister, said: “We are not going to change the Lisbon treaty. It has been approved by the Czech parliament and by the Czech senate in the precise terms in which everybody has accepted it.”

Mr Klaus’s move irritated Fredrik Reinfeldt, the Swedish Prime Minister, whose country holds the EU presidency. “I spoke by phone today with President Klaus,” Mr Reinfeldt said. “I told him this is the wrong message at the wrong time for the EU. I told him clearly it is his ink on the paper that counts, and I do not want this to delay the treaty going through as soon as possible.”

Mr Cameron, wrote to Mr Klaus in the summer stating his intention to hold a referendum in Britain if he won power. But this week the Czech leader suggested that he would not be able to resist signing until May or June.

Even if he manages to delay the treaty for just a few months, it will cause chaos to the EU’s plans. A new European Commission is due to be appointed on November 1, but for legal reasons must wait for the new treaty to come into force. The present commissioners will therefore have to stay in Brussels until Mr Klaus agrees.

EU leaders also wanted the two new jobs created in the treaty, of President of the European Council and EU Foreign Minister, to start in January, with Tony Blair favourite for the President’s post. However, they can be appointed only after the treaty is in force.

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