Brexit has already pushed up the cost of a typical family holiday to Europe by £300 due to the falling value of the pound, the Liberal Democrats have said.
The party said a 10-day trip to the continent for a family of four would now cost £1,609 - up from £1,310.
This is because the euro was typically worth 70p before the June 23 vote, but is now trading at around 86p.
"For some families, this will make a holiday abroad unaffordable this year," Lib Dem leader Tim Farron said.
"This problem is not helped by the failure of Theresa May to say whether she wants Britain to remain in the single market or not.
"I call on her to make clear that she thinks Britain should remain part of the world's most lucrative market and not tie British industry up in endless red tape when it tries to export."
During the referendum campaign, Remain supporters warned that the cost of a typical holiday could increase by £230.
"Brexit campaigners dismissed the figure as scaremongering but it actually proved to be an underestimate," Farron said.