The veteran Tory MP also said that preparations for the single currency had been going on since Margaret Thatcher was prime minister.

Ken Clarke is an enthusiastic supporter of the euro

Later this month, Mr Clarke and Tony Blair will share a platform for the launch of a Britain in Europe Campaign.

He told BBC Two's The Midlands at Westminster programme that "the prime minster's views and mine on the case in principle for the euro are indistinguishable."

He said it is not credible to believe in the EU and not believe in European Monetary Union.

Mr Clarke said that under Mrs Thatcher, the government worked hard to bring about the common market and that "we always contemplated that would lead to a single currency, even during the Thatcher era".

After Labour's drubbing in last month's European elections, Mr Blair was thought to be cooling over his stance towards a single currency.

At one stage, he even admitted the UK population would vote against joining if a referendum were held immediately.

On Friday, Tory leader William Hague attacked the prime minister for leaving doubt about his support for the euro.

He described Tony Blair as the Wizard of Oz for Britain in Europe, saying that all that was behind the curtain was a man with very big media machine.