Five heavyweight Labour politicians, who previously opposed Britain remaining in Europe, have backed David Cameron on the issue, saying the benefits of EU membership outweigh the costs.
Less than a week before the crucial Brussels summit on his renegotiation package, Shadow Foreign Secretary Hilary Benn is among those backing the prime minister's fight to keep us in the EU.
Alongside Benn, former Labour leader Neil Kinnock, ex-home secretary David Blunkett, former foreign secretary Jack Straw and MP Margaret Beckett also backed Cameron in the open letter, printed in the Sunday Mirror.
All five campaigned against remaining in Europe in the 1975 referendum.
Cameron's deal will 'hopefully strength' Britain's relationship with Europe, the Labour heavyweights said
But they said: "Our concern then was that membership would mean a one-way loss of sovereignty and investment. This has proved unfounded.
"The conclusion of the renegotiation will hopefully strengthen this relationship as we make the progressive case for Britain in Europe.
"Leaving would be a huge risk to prosperity, security and the opportunities of future generations. The EU is not perfect and improvement is always worth making, but the benefits outweigh the costs."
Hilary Benn (above), Neil Kinnock (below left) and Margaret Beckett (below right) were among the open letter's signatories
Meanwhil,e top travel figures have warned that leaving the EU could risk tourists' safety and push up flight prices.
EasyJet chief Carolyn McCall suggested a Brexit could herald a return to the days when flying was "reserved for the elite", The Press Association reports.
Peter Long, former boss of the Tui travel group that owns Thomson and First Choice, insisted close co-operation with other EU states was essential to "protect the security of our holidaymakers".
Writing in The Sunday Times, Ms McCall said: "The EU has brought huge benefits for UK travellers and businesses. Staying in the EU will ensure that they, and all of us, continue to receive them.
"How much you pay for your holiday really does depend on how much influence Britain has in Europe."
Ms McCall argued that before the EU overhauled aviation in the 1990s, flying was "reserved for the elite" who travelled on "government-owned airlines between state-controlled airports".
"As a result of Britain's membership, the costs of flights have plummeted, while the range of destinations has soared. That's why easyJet believes the benefits far outweigh the frustrations - and why the UK is better off as part of the EU," she said.
Mr Long, who was in charge of Tui when 33 of its customers were massacred by an Islamist gunman in Tunisia last year, insisted that close co-operation with other EU countries was essential.
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He said witnessing the "human tragedy" after the massacre gave him "many first-hand experiences of seeing how European governments, through their foreign offices, collaborate and work together in a crisis".
"It would not be like that if we weren't in a situation where we were as Europe working together," he wrote in the newspaper.
Mr Long, now chairman of Royal Mail, also cautioned that that Brexit would cause the value of the pound to slump.
"For our customers, that means higher holiday prices and less spending money," he added.
Liam Fox, the Eurosceptic former cabinet minister, accused Downing Street of "scaremongering".
"Those that wish to remain in the EU should make the positive case for the supranational European project rather than frightening people," he said.
The peer Cameron tasked with taking the referendum legislation through the House of Lords has also announced that would be voting to Leave.
Lord Dobbs, who wrote the House Of Cards books, dismissed the premier's renegotiation as "a mouse that barely squeaks, let alone roars".
As the sides in the campaign begin to gear up for an attritional four-month run-in to the likely referendum date of June 23, US secretary of state John Kerry has voiced support for Britain staying in the EU.
Kerry said it was "profoundly" in America's interests that the UK voted to remain in the union.