Gordon Brown has been warned that the Cabinet, and not the Treasury, will decide Britain's policy towards the single currency.

John Reid, the leader of the House of Commons and a close ally of the prime minister, told ITV's Jonathan Dimbleby programme that the decision was "not whether we will join the euro, but when".

"When he [Brown] and the Treasury give an estimate on these things of course it is extremely important. But ultimately these decisions are taken by the Cabinet under the leadership of the first lord of the treasury who is the prime minister...the Cabinet will discuss these matters collectively," he said.

Pro-euro campaigners have today published a letter signed by 25 business leaders which warns that any decision to rule out entry for the rest of this parliament will damage Britain's economy.

Chris Gent, chief executive of Vodafone, Peter Sutherland of BP and former Treasury minister Lord Simon are among the signatories.

And in an interview with today's Guardian, EU commissioner Neil Kinnock calls for a "clear, strong and unmissable signal" of commitment to a euro membership, even if ministers do not name the day.