A quarter of British holidaymakers who travel overseas do not have insurance, new figures show.
Some 25% went abroad without insurance in the 12 months to May compared with 22% during the previous year, according to research for travel trade organisation Abta.
Travellers aged 18 to 24 are the most likely to be uninsured, at 40%, up from 31% last year.
Of those who have embarked on a foreign trip without insurance, more than a third (36%) felt they did not need any cover.
Some 2,043 British adults were surveyed for the study.
Abta warned that the cost of medical treatment while on holiday can run to thousands of pounds.
The family of Craig Lindley, 35, from South Yorkshire, have raised more than £32,000 to help him return to the UK for further treatment after he fell ill in Thailand.
In another case, the loved ones of Dorset grandmother Esther Jones are trying to raise £50,000 after she became unwell while travelling to Australia without insurance.
Abta chief executive Mark Tanzer said: "Every year, we see cases of people falling into difficulty due to travelling without insurance.
"Often their families have to raise thousands of pounds for their treatment or repatriation and that's why it is so worrying to see an increase in younger people travelling without insurance.
"Rather than having to resort to the kindness of strangers, holidaymakers should make sure that they have the right insurance in place."
Susan Crown, from the Foreign and Commonwealth Office (FCO), said: "The FCO cannot pay medical bills if you are hospitalised abroad, nor can we fly you home.
"Take out an appropriate insurance policy and make sure you know what it covers you for. It may feel like an added expense but it's very worthwhile if you compare it to what you could end up paying if something goes wrong on holiday."