Abta says some 38% of Britons (9.9 million) who travelled overseas in the past 12 months have done so with the right travel insurance, took part in activities that may not have been covered, or simply had no cover at all.

More than one in five (22%) people said they had travelled completely uninsured over the past 12 months, while one in four (27%) said they had risked invalidating their insurance by not telling their insurer about pre-existing medical conditions or taking part in activities not covered under their policies.

Ahead of the summer holidays, Abta is calling on holidaymakers to ensure they have travel insurance commensurate with their holiday plans.

The association said the most common reason for people not taking out travel insurance was because they believed it was not necessary, and that schemes such as the European Health Insurance Card (EHIC) would provide adequate cover.

While EHIC provides access to state medical care, it does not include repatriation, which can be severely expensive.

The average cost of a travel insurance claim is just shy of £1,300.

Abta also found around a third of people without insurance (29%) said cost was a factor and that one in seven (15%) said they believed they were adequately insured through their bank.

Mark Tanzer, Abta chief executive, said: “Every year, we see cases of people falling into difficulty due to travelling without sufficient travel insurance.

“While many people are still choosing not to take out travel insurance at all, others are travelling unaware their insurance policy is not protecting them as they expect.

“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 Foreign and Commonwealth Office’s Travel Aware team added: “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. Make sure to take out an appropriate insurance policy and know what it covers. It may feel like an added expense but the costs of not being insured could be many thousands of pounds.

“We’d like to see all British holidaymakers enjoying their holiday safe in the knowledge that they are covered if anything goes wrong.”

Abta has pulled together a five-point travel insurance checklist, and more tips on choosing the right policy, on its website.