Czech President Vaclav Klaus, who has been highly skeptical of increasing the EU's powers, signed the Lisbon Treaty at the Prague Castle hours after his nation's Constitutional Court struck down a complaint against it.

Klaus has been tirelessly attacking and stalling the document, claiming it would hand too much power to European Union institutions in Brussels. He was awaiting the Brno-based court's ruling before deciding whether to endorse it.

"I expected the decision of the Constitutional Court and respect it," Klaus told reporters Tuesday afternoon, but added he vehemently disagrees with the verdict.

"The Czech Republic will cease to be a sovereign state," once the treaty enters into force, he said.

Klaus was the last obstacle to the full ratification of the treaty, which was bogged down in negotiations for almost a decade and has been ratified by all other 26 EU nations.

The Swedish EU Presidency said the treaty will enter into force on Dec. 1.

European leaders welcomed news of the signing.

"President Klaus' decision marks an important and historic step for all of Europe," British Prime Minister Gordon Brown said in a statement.

"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."

German Chancellor Angela Merkel noted during a speech to the U.S. Congress in Washington that, with the new treaty, the EU "will become stronger and more capable of acting, and so a strong and reliable partner for the United States."

"On this basis, we can build stable partnerships with others, above all with Russia, China and India," she added.

Sweden's Prime Minister Fredrik Reinfeldt, who has worked to seal the Lisbon Treaty under the Swedish EU presidency, said he would call for an EU summit as soon as possible.

Klaus' "signature ends a far too long period of institutional focus within the EU," Reinfeldt said in a text message sent from Washington. "It opens up for a more democratic, transparent and efficient Union."

Earlier in the day, the Constitutional Court's chief judge, Pavel Rychetsky, said the Lisbon Treaty "does not violate the (Czech) constitution" and that all formal obstacles for ratification "are removed."

In Brussels, European Commission President Jose Manuel Barroso said he was "extremely pleased" with the court's verdict.

"I hope that we can now move forward as quickly as possible on the nomination of the president of the European Council and vice president of the Commission High Representative," he said, referring to the newly created post of president, who will chair EU summits, and the bloc's new foreign policy chief, who will represent the EU abroad.

Once the Lisbon Treaty becomes law, more policy decisions would be permitted by majority rather than unanimous votes at European summits. Those policies would then increasingly be shaped by the elected parliaments of each nation and the European Parliament, which currently has little say.

EU leaders say such new voting rules are needed to promote stronger policies in combating cross-border crime, terrorism and ecological threats.

Projecting this more decisive EU abroad would be a new fixed-term president _ in place of a decades-old system that rotates the presidency among governments every six months _ and a new foreign minister.

Jerzy Buzek, president of the European Parliament, had appeared confident Klaus would sign the treaty, adding it should now enter into force by the end of the year.

Czech Prime Minister Jan Fischer said he was grateful Klaus finally initialed the document following the court's verdict, saying it sent an important signal to the rest of Europe.

"I am glad that the Czech Republic finally confirmed that it is a trustworthy partner for other member states," Fischer said.

The court was asked to rule by a group of senators who filed a motion arguing the treaty was not in line with the constitution. Last year, the court dismissed a similar complaint.

Failure of the treaty would send the EU into an unprecedented crisis. Negotiators say its reforms _ creating a new EU president post, giving more power to the foreign policy chief and streamlining EU decision-making _ are needed to make the EU more effective.

Last week, EU leaders agreed to Klaus' last-minute demand _ an opt-out from the treaty's Charter of Fundamental Rights in return for his signing of it. Klaus said he was not planning to make any further demands.

The Czech leader asked for the option over worries of property claims by ethnic Germans stripped of their land and expelled after World War II.

But it was considered Klaus had used the demand for the opt-out to try to scuttle ratification of the treaty, which he opposes.

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Associated Press writers Robert Wielaard and Constant Brand in Brussels, David Stringer in London, Geir Moulson in Berlin and Malin Rising in Stockholm contributed to this report.