Tony Blair has given his “100%” backing to Ed Miliband as he attacked David Cameron’s pledge to hold a referendum on Britain’s EU membership.
The former prime minister praised the Labour leader, saying he was “his own man”, and he wanted Labour to win “for the future of our country”.
Mr Blair said Mr Cameron had put “exit on the agenda” and the UK would be a “parochial bystander” outside the EU.
But Mr Cameron insisted he was “putting the country first” in seeking a vote.
Mr Blair won general elections as Labour leader in 1997, 2001 and 2005, and stood down as prime minister in 2007.
BBC deputy political editor James Landale said Mr Blair had “largely stayed out of British politics” since then – but Labour hoped he still had “enough lingering stardust to appeal to some voters”.
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Mr Cameron has pledged to renegotiate a “better deal” for the UK in Europe and to stage a referendum on the UK’s membership in 2017.
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But speaking in his former constituency of Sedgefield in County Durham, Mr Blair said a referendum would be a “huge distraction” and the prospect of possible exit would leave a “pall of unpredictability hanging over the British economy”.
“And the oddest thing of all about having this referendum? The PM doesn’t really believe we should leave Europe; not even the Europe as it is today,” he said.
David Cameron is visiting Scotland, Northern Ireland and Wales in a single day
“This was 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.
“This issue, touching as it does the country’s future, is too important to be traded like this.”
Mr Blair said leaving the EU would leave Britain “diminished in the world” and take the country “out of the leadership game” globally.
Citing the wrangling over Scottish devolution and the growth of the SNP since September’s referendum on Scottish independence, Mr Blair said it was foolish to believe an EU referendum would settle the issue once and for all.
Mr Blair, who has appeared to question some of Mr Miliband’s positioning in the past year, said he “respected” Mr Miliband for putting the “interests of the country first” and opposing a referendum on the Conservative terms.
“He showed on this, as he did on other issues, that he is his own man with his own convictions and the determination to follow them even when they go against the tide.”
Asked whether he agreed with Mr Miliband’s views on the economy, regarded to be to the left of his, Mr Blair said he had always supported the Labour leader and said he had “an excellent set of policies” to deal with the emerging challenges facing Britain.
“I have always had a view that Ed can win and I hope and I am sure he will… I support him 100% to lead our party to victory at the next general election.”
Labour has made its opposition to a referendum a key plank of its appeal to the business community, although firms have warned that although a referendum will cause uncertainty, the EU needs structural reform and the status quo is not acceptable.
The Conservatives have responded by saying Mr Blair has “no credibility” over the EU, suggesting that he “gave away” part of the UK’s rebate on its contributions to the EU when he was in Downing Street.
Former Labour prime minister backs Labour” is one of those news stories that falls into the “dog bites man” variety.
Important, perhaps, to the poor chap with a sore leg but hardly news for the rest of us. So why will we spend so much of today discussing Tony Blair’s intervention in the election campaign?
Speaking in Edinburgh, Mr Cameron said: “Tony Blair is wrong. I’m putting the country first and saying the people of the United Kingdom should be able to have a choice about whether they want to stay in Europe on a reformed basis or leave.
“It’s an in-out referendum by the end of 2017. It’s right for our country, it’s right for Europe too.”
Home Secretary Theresa May said people were aware that it was the Conservatives, not Labour, which were offering a referendum.
“I think, in a sense, the most interesting thing about Blair’s intervention is the fact that he has intervened,” she told the BBC.
“It shows the weakness of Ed Miliband that Labour has had to bring Tony Blair into the fray at this point.”
Mr Cameron, who is visiting all four nations of the UK on Tuesday, earlier appealed to supporters of UKIP – which wants to leave the EU – to “come home” to the Conservatives.
, he said: “There are people who have been frustrated about wanting more changes on immigration, wanting more certainty about the situation in Europe and they can now see that we have listened to those concerns.”
UKIP, which is pushing for a referendum on Europe as early as the end of 2015, has rejected Mr Cameron’s entreaties and welcomed Mr Blair’s intervention as a chance to have a “proper debate” about the UK’s place in Europe.
“It is no surprise to see him pat Ed Miliband on the back for signing up to the corporatist EU agenda,” its leader Nigel Farage said.
“The overall message from the Labour Party is that it wants the British people to trust it yet will not trust the British people to decide how they are governed.”
Lib Dem leader Nick Clegg, who also opposes a referendum in 2017, said he agreed with Mr Blair, suggesting Mr Cameron’s policy on Europe had been driven by a “game of hokey-cokey with the right-wing of the Conservative Party and nothing to do with the national interest”.
Mr Clegg suggested a future Conservative government would be beholden to Eurosceptics in his own party as well as UKIP.
“He (David Cameron) said it this morning: he wants to make a home for Nigel Farage in the Conservative Party…. Just imagine a Conservative Party leadership entirely in hock to Nigel Farage and (Eurosceptic Conservative) Peter Bone.
“It is an absolutely terrifying prospect, but that’s what is perfectly possible and likely without the Liberal Democrats sat around the Cabinet table.”