A senior adviser to chancellor Gordon Brown last night insisted that the timetable for Britain's entry into the European single currency would be based on purely economic grounds.

Speaking at a seminar in London, the chancellor's chief economic adviser, Ed Balls, said: "Too often over the last 100 years we have had decisions taken with a political imperative over economic reasons.

"We cannot let the short term political imperative override long term economic factors. The chancellor's tests have to show that joining would be right not just that day, or even referendum day, but for the future."

The Conservatives are interpreting these comments as a slap-down to pro-euro ministers, including foreign secretary Jack Straw and Europe minister Peter Hain, who have recently called for an early vote on the issue.

The Mirror reveals the secret existence of a policy document, giving a euro "In scenario" of February 2005 if Britain votes "yes" in a May 2003 referendum. The document also shows an "Out scenario" with the UK failing Brown's five tests and/or the government losing a vote on entry to the single currency.