Joining the euro would have been a "huge, huge error", Nick Clegg has admitted.
The Deputy Prime Minister, previously a strong advocate of single currency membership, said that, in "hindsight", Britain had had a lucky escape.
However, he insisted no-one could have predicted the problems which have emerged, and blamed the Germans and French for wrecking the project.
The Liberal Democrat leader was asked about his views on the euro in a round of broadcast interviews at the party's conference in Birmingham. "I think, clearly, with the benefit of hindsight, you can say it would have been a huge, huge error," Mr Clegg replied.
"I don't think anyone could have predicted at the time the euro was created that the rules which were supposed to be in place to ensure that everybody looked after their own financial affairs properly would be so spectacularly ignored and broken.
"I think history will judge the then French and German governments very, very unkindly who, some years ago, basically signalled that the rules could be relaxed because that then sent a signal out to everybody else - oh well, we don't need to keep our house in order. Now I think we are dealing with the consequences."
Mr Clegg said it was very unlikely that Britain would join the euro while he was Lib Dem leader.
"I doubt very, very much that, during my political lifetime, certainly as leader of the Liberal Democrats, that we will see the UK enter into the euro," he said.
Senior Lib Dem Sir Menzies Campbell later claimed that the party's stance on the euro was "right at the time".
He told BBC News: "I don't think the problem was in the scheme of the European single currency - the problem was enforcement. People assumed, including myself, that countries that signed up to it would meet their obligations under the scheme and palpably they have not."