British Prime Minister Mr Tony Blair's instinct remains to press for a referendum on British membership of Europe's single currency before the next election, his key ally and confidant Mr Peter Mandelson said yesterday.

But Mr Mandelson signalled a vote was more likely next year than this.

The Chancellor of the Exchequer, Mr Gordon Brown, has pledged to deliver his assessment of the economic case for euro entry by the first week of June.

With the UK economy set to grow twice as fast as that of the euro zone this year, economists say it will be difficult to report the unambiguous convergence that Mr Brown has always said is necessary for the cabinet to recommend switching currencies.

But Mr Mandelson, a pro-euro former cabinet minister who is still close to Mr Blair, told BBC Radio the premier sees real economic and political costs to remaining outside the euro zone.

"I believe he considers Britain...will miss out economically, primarily, but also politically if we remain outside the euro indefinitely," he told BBC Radio. "I think his instinct will be to go for a referendum in this parliament."