Tony Blair enters the election campaign today by attacking David Cameron for offering an in-out referendum on the EU which he claims will plunge Britain into the worst instability since the World War Two - and praising Ed Miliband for "real leadership" on the European issue.

Blair, an ardent pro-European, will accuse Cameron of pandering to Ukip by offering the referendum to shore up the Tory vote. He will say Cameron does not genuinely want to leave Europe as it stands – he is doing it only for short-term political gain.

The offer of a referendum “was a concession… 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”.

Blair was due to issue his warning in a speech this morning in Sedgefield, the County Durham constituency he used to represent as MP. He will say: “There is, in my view, a complete under-estimation of the short-term pain of negotiating exit. There would be a raft of different treaties, association agreements and partnerships to be dis-entangled and re-negotiated.

“There would be significant business uncertainty in the run-up to a vote but, should the vote go the way of exit, then there would be the most intense period of business anxiety, reconsideration of options and instability since the war.”

Blair also will praise Ed Miliband’s “real leadership on the EU”, saying he admires the way he had shown “he is his own man with his own convictions and determined to follow them, even when they go against the tide”.

Of course, there’s a catch. James Landale, the BBC’s deputy political editor, reminded listeners to Radio 4’s Today programme that “Tony Blair is less popular than he once was and few pretend he and Mr Miliband agree on everything… but Labour hopes that their three-time winner still has enough lingering stardust to appeal to some voters”.

Even the left-leaning Guardian, which was given the “exclusive” about Blair’s intervention, notes that it could prove a double-edged sword for Miliband.

Patrick Wintour, the paper’s political editor, says: “Such is the fragility of political reputations, and arguably so mismanaged has the post-premiership career of Tony Blair been, that the former Labour leader’s intervention in the general election is a matter of anxiety in Labour circles.

“Far from being a returning conquering hero, Tony Blair is the secret weapon that half the Labour Party would like to stay secret.”

Blair is also a potential turn-off for Lib Dem voters, argues Wintour. Some within Labour worry that his presence “will act as a reminder to the 2010 Liberal Democrat vote, now back in the Labour fold, of why they first deserted Labour and turned to the "left-wing” Nick Clegg”.

"Soft Tories", on the hand, have a soft spot for Blair - and, says Wintour, "it is among Tory switchers that this election could yet be decided".