Czech President Vaclav Klaus gave Tuesday the long-awaited final seal of approval to the European Union's reforming Lisbon Treaty, clearing the way for the bloc to find a new president.

The ardent eurosceptic became the last EU leader to sign the landmark document into law after the top Czech court ruled that it was in line with the country's constitution.

Even after announcing that he had signed the treaty, Klaus remained critical of the document.

"I cannot agree with its contents since after the validation of the Lisbon Treaty... the Czech Republic will cease to be a sovereign state," he told reporters.

Swedish Prime Minister Fredrik Reinfeldt, whose country holds the rotating EU presidency, said the treaty would come into force on December 1.

The signatures untie the 27-nation EU's hands to appoint a new European Commission -- its executive arm -- as well as fill the freshly-created posts of president and foreign affairs supremo.

Reinfeldt said the signing ended a "far too long period of institutional focus within the EU" and opened up the way "for a more democratic, transparent and efficent union."

He said he would call an EU summit shortly and "begin name consultations" among the 27 member states to find a new top boss and chief diplomat.

The Lisbon Treaty is designed to smooth the workings of the EU, which has almost doubled in size to 27 nations since a swathe of ex-communist nations including the Czech Republic joined in 2004.

The delay in implementation has hampered the work of the European Commission, whose mandate expired at the end of October, and put on ice the appointment process for the two top EU jobs.

Belgian Prime Minister Herman van Rompuy has been mentioned as a consensual candidate for president after support for former British Prime Minister Tony Blair proved weak at an EU summit last week.

Britain's current Prime Minister Gordon Brown welcomed Klaus's decision to sign, saying it "marks an important and historic step for all of Europe."

"Today is a day when Europe looks forward, when it sets aside years of debate on its institutions, and moves to take strong and collective action on the issues that matter most to European citizens: security, climate change, jobs and growth," Brown said.

European Commission chief Jose Manuel Barroso said he believed that "the transformational potential that is there, the new external profile for the European Union, will be felt immediately."

But Klaus, who refuses to fly the EU flag at his residence, remained unconvinced.

"Good cloudy afternoon," Klaus, wearing a black suit and tie, said in a grim voice before telling reporters he disagreed with the constitutional court ruling that paved the way for him to sign.

"I expected the court ruling and I respect it, although I disagree with its contents and reasoning," he said, calling the verdict "a biased political defence of the Lisbon Treaty."

Czech lawmakers approved the treaty earlier this year, but Klaus refused to sign pending the top court's verdict on a complaint by pro-Klaus senators. The court had in any case ordered him to wait until it ruled.

After Ireland's two referendums on the treaty and with renewed agonising over Europe in Britain, EU leaders were anxiously waiting for Klaus to end the impasse.

At a summit last week, they agreed to give Prague an opt-out from parts of the treaty, demanded by Klaus in what critics saw as another attempt to delay the ratification.

Klaus had asked for an exemption to ensure the treaty would not allow ethnic Germans forced out of his country after World War II on charges of Nazi collaboration to take legal action to reclaim their property.

Backers of the treaty got more good news on Tuesday as Britain's eurosceptic opposition Conservatives said they had dropped plans to call a referendum on the treaty if they took power next year.