Two top travel industry figures have warned that leaving the EU could risk tourists' safety and push up flight prices.
Less than a week before a crunch Brussels summit on David Cameron's renegotiation package, easyJet chief Carolyn McCall suggested a Brexit could herald a return to the days when flying was "reserved for the elite".
Meanwhile 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".
And five previously Eurosceptic Labour figures, including shadow foreign secretary Hilary Benn, have thrown their weight behind Mr Cameron's deal.
In an open letter, they said the EU was "not perfect" but leaving would be a "huge risk".
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.
He said witnessing the "human tragedy" after the Tunisian 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.
Alongside Mr Benn, former Labour leader Neil Kinnock, ex-home secretary David Blunkett, former foreign secretary Jack Straw and MP Margaret Beckett also backed Mr Cameron in the letter, printed in the Sunday Mirror.
All five campaigned against remaining in Europe in the 1975 referendum.
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."
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 Mr Cameron tasked with taking the referendum legislation through the House of Lords has also announced that would be voting to Leave.
Lord Dobbs, creator of House Of Cards, 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.
Mr Kerry said it was "profoundly" in America's interests that the UK voted to remain in the union.
In his last big speech before the Brussels summit on Thursday and Friday, the Prime Minister told an audience in Hamburg on Friday night that Europe had to "stand together" against threats such as Islamic State (IS) and Russian aggression.
He also appealed for Germany's help in finalising his package of reforms, stressing the countries' shared interests and values.