Referendum threat from Conservative eurosceptics

Tory MP Douglas Carswell said that he and colleagues are planning to take advantage of a minor technical change to the Treaty to trigger a public vote.

The move would be a headache for Prime Minister David Cameron, who dropped his "cast-iron guarantee" of a referendum on the Treaty after it was ratified into European law last year.

And it would provide an early test of the unity of the new Tory coalition with the strongly pro-European Liberal Democrats.

Mr Cameron's coalition agreement with Lib Dems committed the new Government to a "referendum lock" requiring public approval for any proposed future Treaty that transferred areas of power or competences to Brussels.

The Foreign Office confirmed primary legislation will be needed to ratify a technical amendment to the Treaty rebalancing the numbers of MEPs from each member-state in line with last year's agreement.

The Lisbon Treaty provided for 12 member states, including the UK, to have a total of 18 additional MEPs, while Germany would lose three. However, this could not be put into effect at last year's European Parliament election because the Treaty was not yet in force.

Bringing in the extra MEPs before the next elections in 2014 would require a technical amendment, and the European Council is expected to decide in June whether to press ahead with the change.

"If this protocol is agreed, primary legislation will be needed in the UK to ratify this technical amendment to the Treaty," said the Foreign Office.