Former Tory chancellor Kenneth Clarke says he does not think Britain could join the euro with "complete security and confidence" for 10 years.
Clarke: Staunchly pro-euro in past
In an interview with the journal Central Banking, he said the single currency had so far failed to deliver increased productivity in Europe.
The pro-European's remarks are seen as the strongest hint yet that he intends to stand for the Tory leadership.
Mr Clarke's pro-euro stance had been seen as an obstacle to him standing.
Mr Clarke, who came second in the 1997 and 2001 Conservative leadership contests, has been backed to be leader by ex-deputy prime minister Lord Heseltine.
There have been suggestions that he could form the senior part of a "dream ticket" with 38-year-old shadow education secretary David Cameron.
But Mr Cameron's friends have said they have ruled out doing such a deal, pointing out their differences on a range of issues, including Europe.
In the interview, however, Mr Clarke says that he now accepts the likelihood of Britain signing up for the euro in the near future is remote.
"I do not think there has been a time when the British could have joined with complete security and confidence. I doubt it is possible for 10 years or more," he said.
He concedes that the single currency has so far proved a disappointment to its supporters.
"I thought it would lead to increased productivity, efficiency and living standards and stimulate policy reforms. On that front so far it has been a failure," he said.
He says he is also concerned about the way some eurozone members are failing to show the fiscal discipline necessary to make the euro work.
"I am beginning to worry considerably where Italy is going. The Italian government is utterly oblivious of the need to retain some fiscal discipline," he said.
"It is still running a kind of family capitalism without paying any heed to the level of wages or other costs."
Mr Clarke also argues that the European Central Bank, which sets interest rates for the eurozone, needs to "rethink the role it plays in the economic life of the nations it serves".
"It must stop imitating the Bundesbank, which was an institution suited for the 1960s," he said.
But he praises UK Chancellor Gordon Brown's decision to give the Bank of England the operational independence to set interest rates in Britain.
"If by some unlucky chance I became chancellor of the exchequer tomorrow, I would immediately confirm my predecessor's arrangements," he said.
He reaffirms his belief that the European Constitution is effectively dead following the rejection of it by voters in France and the Netherlands.
"There is no way of rescuing the treaty, although I was in favour and the sooner we can make a reality of economic reforms in terms that are seen by the public as contributing to their economic wellbeing, the better," he said.