Over 50% of British travellers who wound up needing medical assistance abroad found their European health insurance card (EHIC) did not cover all their expenses, figures show.
According to Sainsbury's Travel Insurance, cardholders paid an average of £386 to settle their bills.
The survey found of those who had to pay extra, 51% did not - as they may have been entitled to - claim any of this back when they returned to the UK.
Based on responses from 1,011 adults, the poll showed 10% of those who had travelled to Europe in the last 12 months had received medical treatment with their EHIC card.
Of the cases, 12% involved flu-type bugs, 10% were heat-related, 8% involved winter-sports activities and 3% involved heart attacks.
The survey also showed that 8% of people who had an EHIC card had not taken out any other insurance as they thought they were already totally covered.
Sainsbury's Travel Insurance manager Sam Marrs said: "Many people wrongly overlook travel insurance because they think their EHIC will cover them when they are abroad.
"But it will not cover you for private medical care, nor will it pay for you to be airlifted off a ski slope or repatriated to the UK. By not having travel insurance, many people are taking on a great risk when they travel in Europe."
© Press Association 2008