The prime minister has failed to learn the lessons from Black Wednesday, Lord Lamont has claimed.

John Major's former chancellor, who oversaw Britain's exit from the Exchange Rate Mechanism a decade ago, warned it would be "folly" to join the euro for political reasons.

"The prime minister has said it would be folly to refuse to join the euro for political reasons," he said.

"It would be a much greater folly to do the reverse and for political reasons join the euro regardless of the risks.

"Many of the political arguments now advanced in favour of Britain membership of the euro were wheeled out at the time of the ERM, and the economic reasons were seen as less important than the political price to be paid if Britain stayed out."

Lamont predicted that history may be about to repeat itself and claimed the prime minister even claimed Blair was prepared to ignore chancellor Gordon Brown's famous five economic tests.

He claimed those tests would be fought over by both sides of the single curency divide.

"It is impossible to see how the government's own test of cyclical convergence could ever be unambiguously satisfied," he said.

"Clearly there are huge risks for Britain in joining the euro and there is no way the evidence in the near future can be clear and unambiguous."