Tony Blair will enter the election campaign later with an attack on David Cameron’s pledge to hold a referendum on Britain’s EU membership.
The former PM will accuse Mr Cameron of trading national interest for political advantage under pressure from UKIP and anti-European Conservatives and media reports BBC.
Mr Blair will say Labour’s Ed Miliband has shown “real leadership” by resisting pressure over a referendum.
But the Tories said his intervention was a sign of Mr Miliband’s “weakness”.
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”.
“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, who has appeared to question Labour’s positioning in the past year, will say leaving the EU would leave Britain “diminished in the world”, adding that he “respects” Mr Miliband for putting the “interests of the country first”.
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.