In a speech in London on the future of Europe, the former prime minister will say that the EU is undergoing its gravest crisis since its formation after the second world war. He will even say that some EU countries are undergoing an existential crisis.
Amid concerns among some pro-Europeans that Cameron will struggle to shape the future of the EU, by using the eurozone crisis to demand the repatriation of social and employment laws, Blair will warn that Britain could set itself back by 20 years if it "falls away" from shaping the debate.
In a speech to the Business for Europe group, Blair will 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. Then it was about a continent ravaged by war in which Germany had been the aggressor and Britain the victor.
"Today it is about a world in which global geopolitics is undergoing its biggest change for centuries. Power is shifting west to east. China has emerged finally, with its economy opening up which will grow eventually to be the world's largest. Its population is three times that of the whole of the EU. India has over a billion people. Brazil is two times the size of the largest European country, Indonesia three times and there are a host of countries including Russia, Turkey, Mexico, Vietnam, the Philippines and Egypt larger today than any single EU nation."
Blair, who spends an increasing amount of time in China, believes many British politicians have failed to realise the implications of the power shift east. He wants to make clear that Britain cannot hope to be an influential player unless it is an enthusiastic member of one of the world's great trading blocs.
The former prime minister, who rarely made speeches about Europe in Britain when he was in office, is making his intervention shortly before Cameron sets out his vision for Britain's role in the EU. The prime minister is expected to suggest that a Conservative government would hold a referendum on renegotiated terms of membership after the 2015 general election. Cameron plans to withhold Britain's support from an EU treaty on new governance arrangements for the eurozone unless the other 26 leaders agree to repatriate some powers to Britain.
Blair warned in a speech in Berlin last month of the dangers of playing "short-term politics" with the EU. He challenged the acceptance by Cameron and Osborne of a two-speed Europe. The prime minister and chancellor have said it is right for the 17 members of the eurozone to create a set of governance arrangements for the single currency.
Blair said in Berlin: "I can almost feel the relief in some euro-federalist quarters and amongst most Eurosceptics at the prospect of a two- or three- speed Europe. But I would give a stark warning: if eurozone structures end up with a Europe that is fundamentally divided politically as well as economically, rather than a Europe with one political settlement that accommodates different levels of integration within it, the EU as we know it will be on a path to break-up."
He repeated his call for a directly elected president of the European commission or European council. Blair had been keen to be appointed president of the European council in 2009. The post went to Herman Van Rompuy, the former Belgian prime minister, with the support of Angela Merkel.
guardian.co.uk © Guardian News and Media Limited 2010
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