Holidaymakers are being warned not to leave insurance as an "afterthought" - as they could be left with a bill for emergency medical treatment abroad that is higher than the price of a house.
The Association of British Insurers (ABI) said insurers help more than 3,000 travellers every week who need emergency medical care while abroad.
Yet it said previous research has suggested around one in five go abroad without travel insurance.
While the average travel insurance claim is just over £700, emergency medical and repatriation costs when overseas can be much higher.
The ABI said some emergency treatment can be much more expensive than the average UK house price, at around £211,000.
Emergency medical bills in the United States can be considerable, it said. For example, an insurer recently paid a medical bill of £322,000 for treating a swollen blood vessel in the brain.
Other medical bills faced by Britons covered by travel insurance include £300,000 for a tourist who suffered multiple injuries after falling from a waterfall in Thailand, £40,000 to pay for the medical costs of a traveller who was bitten by a mosquito in Indonesia and contracted Dengue fever, £31,000 for treating a broken leg in Nepal that became infected and £11,000 for a holidaymaker with a brain tumour in Spain.
Mark Shepherd, the ABI's manager, general insurance, said: "Anyone travelling overseas should always take out appropriate travel insurance for the duration of their trip, and declare medical conditions when they take out their policy.
"A valid European Health Insurance Card (EHIC) when travelling in Europe is also strongly recommended. Though not a substitute for travel insurance, the EHIC is free and provides access to state-provided healthcare on the same basis as a resident."
He added: "Travel insurance should not be an afterthought, but the first thing you arrange after booking any overseas trip."