Tony Blair has said the UK will face an "interesting choice" over whether to join the euro if the currency's current crisis is resolved.
The former prime minister told the BBC he believed the UK should still be keeping open the option of joining it.
He said that looking at the "broad sweep of history" in the long term "the European integration project" was going to go ahead, "like it or not".
The UK, as a "small island nation", had to be part of it to have influence.
Mr Blair said that the only thing that would save the single currency now was to have a "grand plan" where Germany was ready to commit its economy fully - "treating the debts of one as the debts of all".
This would be difficult for Germany, he said, and would have to be in return for other countries having "precise, deliverable" programmes of change and reform that could restore European competitiveness.
As well as economic changes, political change was also inevitable with Europe needing reform of labour markets, pensions, welfare, public services the role of the state, he said.
He told BBC One's Andrew Marr Show that his former chancellor Gordon Brown had "always been right" on the economic case against the UK joining the euro when Labour was in power.
But Mr Blair said he had always believed that it was important to be supportive of the project politically so the UK could join when the conditions were right "and I still think that's the case".
Asked if that meant he still thought the UK should join the euro one day he said: "Look, if they sort it all out and Europe moves forward again then Britain's going to have a very interesting choice in the future."
Mr Blair, who was prime minister from 1997 to 2007, told the same programme that he was still interested in public service and would have been happy to have remained as prime minister, or to have got the job of European president.
On the recent controversy of tax avoidance schemes, Mr Blair said he thought the mood had changed, saying that in tough times people wanted to know everyone was paying their fair share.
Mr Blair, who said he had made 86 trips to the Middle East since leaving office, said he believed that the Arab Spring was actually an "Arab revolution" and "would sweep across the whole region and beyond".
He said the bad news was that in the short term there would be pain for people in the Middle East as they attempted to develop democracy after the overthrow of dictators, but in the long term it would be good news because it showed people wanted freedom.
The former PM also said there was nothing new in the suggestion, in Sunday's Independent, that he had stopped the attorney general telling the cabinet the full detail of the legal concerns ahead of the Iraq war in 2003.
"No, it's absolutely not true... the notion that Cabinet never discussed this issue is absurd... there is no great hidden conspiracy about this, it was a decision (to go to war). Now some people agree with it, some people disagree with it," he said.
He added that when people "looked at the Middle East today" he thought that in the "broad sweep of history, people will take a different view of it".