Tony Blair has dramatically intervened in the General Election campaign to warn of economic chaos if Conservative victory leads to an in/out referendum on Britain's EU membership.
The former prime minister voiced his "100%" support for Ed Miliband as he warned that exit from Europe would "leave Britain diminished in the world" and even the prospect of a public vote in 2017 would create "significant business uncertainty".
But David Cameron said Mr Blair was "wrong" and accused him of not trusting the British people.
Speaking in Belfast during a whirlwind tour of the four nations of the United Kingdom, Mr Cameron said: "I want changes in Europe but then, unlike Tony Blair, I will trust the people in an in-out referendum.
"You cannot ignore the will of the people as Tony Blair thinks we should - and it is not just him, it is Ed Miliband."
Mr Miliband visited Bristol to launch an attack on the Conservatives' stewardship of the NHS, on the day that official figures showed waiting times in A&E departments had slumped to their worst level in a decade.
The Labour leader welcomed Mr Blair's intervention, and brushed off questions about why the pair had not shared a platform, saying: "He can speak for himself."
Tories said that it was a sign of Mr Miliband's "weakness" that Labour felt it needed to draft its former leader in.
But Mr Blair said he was speaking out because of his concern at the damage EU withdrawal could do to Britain's standing in the world, and voiced his "respect" for Mr Miliband's leadership in standing up to pressure to match Mr Cameron's referendum promise.
In a high-profile speech in Newton Ayliffe, during a visit with wife Cherie to his former Sedgefield constituency, Mr Blair said : "I believe passionately that leaving Europe would leave Britain diminished in the world, do significant damage to our economy and, less obviously but just as important to our future, would go against the very qualities that mark us out still as a great global nation."
He said he did not believe Mr Cameron even wanted to quit the EU, but said the PM had offered a renegotiation of UK membership, followed by a public vote, as "a concession to party, a manoeuvre to access some of the Ukip vote, a sop to the rampant anti-Europe feeling of parts of the media".
If the Tories won on May 7, Europe would be a "distraction" from the work of government for a number of years, and would cost Mr Cameron "more energy ...more sleepless nights" than any other issue, he warned.
"The Tory campaign talks of chaos should Labour win," said Mr Blair. "Think of the chaos produced by the possibility, never mind the reality, of Britain quitting Europe. Jobs that are secure suddenly insecure; investment decisions postponed or cancelled; a pall of unpredictability hanging over the British economy.
"And for what? To satisfy the insistent Euro-phobia of a group who will never be satisfied."
The former PM denounced the "mean-spiritedness" of the nationalism espoused by Ukip, and said: "We know what this movement to wrench us out of Europe is based on. You can see it whenever (Nigel) Farage swiftly moves the debate to immigrants.
"National pride is a great thing. Nationalism, as a political cause in the hands of parties like Ukip, is almost always ugly and, despite being wrapped in the garb of high-sounding phrases, can never disguise its essentially mean spirit."
But Mr Farage said Ukip did not represent nationalism but "nation-ism" - the right for people to live in nation states rather than a federal superstate.
During a campaign visit to Cannock, Staffordshire, the Ukip leader said "bravo" to Mr Blair for putting the issue of EU membership at the heart of the election debate, adding: "I'm very pleased he talked about it and very interested about his comment about David Cameron - that he's been forced into promising a referendum he doesn't really believe in."
Senior Tory Michael Gove said Mr Blair had little credibility on Europe because he wanted Britain to enter the euro and wanted more integration with the EU.
The Chief Whip told BBC Radio 4's PM: "I think that Tony Blair was reliving some of his own psychodrama.
"The truth is that he was in favour of a referendum before he was against it.
"He's occupied different positions on the European question and he looks back on his time in power and he regrets the fact, I'm glad I have to say, that we did not integrate even further into the European Union.
"Tony Blair, I think, has limited credibility on this issue because he was one of those who argued passionately for our entry into the single currency and I don't think there's any respectable politician now who would want to waste their breath arguing that we should enter the euro."