The treaty was drawn up to streamline decision-making in the EU, and is a watered-down version of a draft EU constitution rejected four years ago.

Among its measures, it creates a European Council president and alters the way member states vote. The treaty could now come into force as early as December.

The Lisbon Treaty's supporters say it will allow the EU to operate more efficiently and give it greater influence in world affairs. Critics say it will cede too many national powers to Brussels.

Speaking in Washington ahead of an EU-US summit, European Commission President Jose Manuel Barroso welcomed the removal of the "last hurdle" to the treaty's passage.

"I think that the transformational potential that is there, the new external profile for the European Union, will be felt immediately," he said.

In the UK, William Hague, the shadow foreign secretary for the opposition Conservatives, said it would no longer be possible to hold a referendum on the treaty if his party won a general election next year.

The party had previously argued that the treaty should be put to a popular vote.