The European Union cannot function effectively with 25 members under its present system, the prime minister has said.

Speaking at his monthly press conference, Tony Blair said he would continue to campaign in favour of the new constitution.

A referendum on reform of the EU's organisation is expected to be held in Britain in the spring of 2006.

Polls suggest British voters are the most sceptical in Europe about the new constitution, raising the prospect that the UK could be the only country to reject the agreement.

But Blair said on Wednesday that he would be "enthusiastic about our acceptance of the constitutional treaty".

"It is necessary for Europe to function at 25, 27, 28 and more members," he said.

"There is absolutely no way that Europe can function effectively under its present system.

"One thing that is becoming really clear at every European summit that I now attend where you have got 25 different countries and their foreign ministers sat around the table - there is now way you can do business in the most effective way with such a large number of people."

Blair added that plans for a full time presidency of the European council, replacing the present system where the presidency rotates between national governments, was "absolutely essential".