An estimated 10 million Britons travelled abroad without adequate insurance in the past 12 months, a survey has revealed.
Research for travel trade organisation Abta found that almost two in five (38%) holidaymakers were not properly covered if they had got into difficulties overseas.
A poll of 2,015 adults revealed that 22% went on holiday completely uninsured, while 27% risked invalidating policies by not declaring pre-existing medical conditions or taking part in activities which were not covered.
The most common reason given for not buying travel insurance was a belief that it is unnecessary, with many believing the European Health Insurance Card (EHIC) provides sufficient protection.
The card provides access to state medical care but does not pay for seriously ill people to be repatriated to the UK.
Abta chief executive Mark Tanzer said: "While not declaring existing medical conditions or taking part in activities that aren't covered are easy mistakes to make, they can be very costly, leaving holidaymakers and their families with expensive medical bills which run into thousands of pounds.
"I would urge all holidaymakers to make sure they take out travel insurance and check that it covers their circumstances and holiday plans."
Susan Crown, from the Travel Aware team at the Foreign and Commonwealth Office, said: "People are risking thousands of pounds in medical bills by travelling without an insurance policy that covers them for everything they want to do abroad.
"It's important to know that the FCO cannot pay medical bills if you are hospitalised abroad, nor can we fly you home."