Britain cannot afford to "fall away" from the European Union at a time when the continent faces a moment of "existential crisis" due to the global shift in power from west to east, former prime minister Tony Blair will warn today.
In a speech to a business audience in London, Mr Blair will say that the rationale for a European Union is stronger now than at any time since its formation 60 years ago, and warn that if the UK follows the path of withdrawal, it may end up spending the next two decades trying to get back in.
Despite the current difficulties caused by instability in the euro, he will argue that Britain needs the "heft" provided by membership of the world's biggest single market bloc to maximise its opportunities internationally.
The rise of the new economic powers represents "the biggest change for centuries" in geo-politics, and presents Europe with a crisis as serious and as pivotal as in the post-war years when the EU was first created, Mr Blair will say. While the challenge then was to secure peace for a war-ravaged continent, now it is to secure a share of power in a changing world.
With economic power increasingly shifting towards emerging giants like China, India and Brazil, the UK cannot afford to lose the ability to shape the future of Europe, he will warn.
The former PM's intervention comes just days after the leaders of the 27 EU members, including Prime Minister David Cameron, failed to agree on a seven-year budget for the bloc.
And it follows suggestions from Conservative vice-chairman Michael Fabricant - firmly rejected by the Tory leadership - that Mr Cameron should consider offering an in/out referendum on EU membership as part of an electoral pact with the UK Independence Party.
Mr Blair's speech is being hosted by the Business for New Europe group, which backs UK membership of a reformed, open and competitive free-market EU.
It was being seen as an attempt to rally business support behind the cause of EU membership in order to halt the drift towards withdrawal reflected in the opinion polls as well as in Ukip's improving electoral fortunes.
Mr Blair is expected to say: "The truth is the rationale for Europe today is stronger not weaker than it was back 60 years ago when the project began. But it is different. Then the rationale was peace. Today it is power."