Research by the Foreign & Commonwealth Office (FCO) published today reveals a third of those who fail to take out travel insurance choose not to because they are only going on a short break.
Just over a quarter claim it is too expensive and nearly one in five believe they do not need to as they are visiting family and friends.
One in five with a policy are willing to risk invalidating it by not declaring an existing condition.
Fifteen per cent admit that they would not update their policy at all if they developed a medical condition or were prescribed new drugs for an existing condition.
Mark Simmonds, Minister for Consular Policy, said: “‘It won’t happen to me’ or ‘I’ll be fine’ are risky assumptions to make when deciding whether or not to take out comprehensive travel insurance.
“Our consular staff around the world deal with thousands of cases each year that prove that things can and do go wrong.
“Being prepared can mean the difference between the holiday of a lifetime and a holiday from hell.
“We will do everything we can to support people who find themselves needing medical assistance or treatment abroad, but the FCO cannot pay medical bills or fund medical repatriation back to the UK.”
According to the Association of British Insurers (ABI), the cost of medical treatment is often significantly underestimated by people travelling abroad.
It said a claim in the United States for blood pressure and cholesterol-related medical emergencies, such as heart attacks and strokes, could cost up to £100,000.
A spokesperson said: “Travel insurance is a must for all holidaymakers.
“Under the Age Agreement which we developed with Government and the British Insurance Brokers’ Association, older people who are unable to find cover at the first firm they approach should be directed to an alternative provider who will be able to offer insurance or to a signposting service.”