Foreign Secretary David Miliband insisted the controversial treaty would improve the European Union's ability to act internationally.

The treaty will create a new EU High Representative for Foreign Affairs and Security Policy and a permanent president of the European Council – changes that critics claim will damage Britain's ability to manage its own international affairs.

It will also create a new European External Action Service, which Eurosceptics claim will act as an EU diplomatic corps.

Mr Miliband said the EU would act "not as an alternative to UK foreign policy but as one means for its implementation" and the High Representative would give Europe a "more coherent voice internationally".

During a Commons debate on the foreign and defence policy elements of the treaty, he said where EU governments worked and agreed with each other "we are stronger for it and the world is better for it".

He added: "What this treaty will do is enhance the efficiency, effectiveness and coherence of the current arrangements."

The High Representative will be appointed by national governments and responsible to them through the European Council. "It will remain the Council – not the High Representative – that takes decisions.

"In short, Britain will continue to decide on its own foreign policy and where we agree with others there can be a common European role in helping to deliver it."

On defence policy, the treaty will also commit the EU to mutual assistance in the event that one member state is attacked.

Meanwhile, Shadow Foreign Secretary William Hague was today expected to join EU referendum campaigners in Pudsey as an unofficial poll in the constituency nears its end.

Voters in the constituency have until Monday to take part in the poll which is organised by pressure group I Want A Referendum. Organisers hope to use the results to press Gordon Brown to hold a national referendum into whether the Lisbon Treaty should be ratified.