Family holidays to Europe could be £230 more expensive on average if Britain leaves the EU, David Cameron will say as he maintains an offensive over the claimed economic threats of Brexit despite a Tory backlash.
The Prime Minister will draw on analysis by the Treasury suggesting a 12% slump in the value of the pound to warn tourists of sharply rising bills for accommodation, food, drink and other costs abroad.
But the PM's latest salvo in the increasingly bitter referendum battle will intensify allegations of scaremongering from the Leave camp, which strongly disputes the Whitehall calculations as negative propaganda.
His warning that leaving the EU would plunge the UK into a year-long recession and cost up to 820,000 jobs sparked reports that Tory MPs - one of whom, Marcus Fysh, called the Treasury dossier "specious b******s" - planned a vote of no confidence.
With less than a month to go until polling day however, the Remain campaign was boosted by a poll showing older voters, men and Conservative backers all moving their support away from Brexit.
The Daily Telegraph said its latest survey of 800 people by ORB International gave the pro-EU side a 13-point lead - by 55% to 42% - among those certain to vote, thanks in part to dramatic shifts among the three key groups.
Over-65s and male voters - seen as the most keen on quitting the EU - backed Remain by 52% to 44% and 55% to 42%, the newspaper said, with Conservatives, who favoured Brexit by 60% to 34% in March, now 57% to 40% for Remain.
According to Government calculations, by 2018 the weaker pound will mean four people on a nine-night break to Spain could pay £225 more, with eight nights in France up £210, a fortnight in the US £620 and 10 nights in Portugal £325,
A defiant Mr Cameron enlisted the support of a budget airline and mobile phone giants to back suggestions cheap flights and lower-cost overseas calls would also make holidays more expensive in the event of a divorce with Brussels on June 23.
Free healthcare is also at risk for tourists in EU countries and new limits on duty free would jeopardise the "booze cruise", he will suggest.
"All the evidence points to the value of the pound falling after a vote to leave the EU. A weaker pound means people's hard-earned savings won't go as far on holidays overseas.
"The choice facing the British people on 23 June is increasingly clear: the certainty and economic security of remaining in the EU, or a leap in the dark that would raise prices – including the cost of a family holiday."
Carolyn McCall, chief executive of easyJet, backed the PM over rising prices and the threat to cheap flights, saying its passengers would be "better off in Europe".
"If the UK were to vote to leave the EU any new, more restrictive aviation arrangements would add cost and therefore fares would rise," she said.
On roaming charges - which have been cut and are set to be abolished from June 2017 - Number 10 was backed by BT Group chief executive Gavin Patterson and Vittorio Colao, chief executive of the Vodafone Group.
Mr Patterson said: "Voters need to think very carefully before turning their backs on an institution that helps to ensure benefits like that are delivered."
Both said remaining in the EU would be better for their businesses and for UK jobs.
Mr Colao added: "Britain will benefit from being part of a borderless European Single Digital Market as it will create new opportunities for economic growth.
"Consequently, we believe it is better to be a shaper and leader from within, rather than being just a commercial neighbour."
Mr Cameron also risked riling opponents after it was confirmed that he will not take part in any head-to-head televised debates in the run-up to the vote.
In the latest announcement of on-screen line-ups, the BBC said the PM would face a public grilling in a Question Time special on June 19 - four days after pro-Brexit cabinet colleague Michael Gove appears in the same format.
The PM has declined to engage in any "blue-on-blue" confrontations with fellow Conservatives.
Labour will seize on the announcement as further evidence of a lack of female presence in the public debate as Harriet Harman asks watchdog Ofcom to ensure programmes are less male dominated.
Labour has called for broadcasters to be forced to give more airtime to female voices on each side of the EU referendum, warning women are being "pushed out" of the debate.
Ms Harman said the argument over Britain's future membership was "too important to be left to men" and has written to Ofcom calling for action from the regulator to ensure a better balance.