The holiday season looms, and as a surprisingly high proportion of travellers discover, a foreign holiday can rapidly turn into a nightmare if health problems strike.

Yet thousands go away every year without adequate protection.

For anyone heading to the Continent, the European Health Insurance Card (EHIC) is essential, but a fifth of UK adults have never heard of it, even though one in ten have relied on it to get treatment at reduced cost after falling ill or having an accident.

A similar proportion of holidaymakers admit they never bother to take out travel insurance before going away, despite the fact that without it, if something goes wrong, they could be faced with a bill for tens of thousands of pounds.

Meanwhile, more than half of those who do have an EHIC don't understand the limits of the cover it provides, leading them to underestimate the importance of getting the right travel policy.

According to website Gocompare.com, half of UK adults who have travelled abroad wrongly believe the EHIC entitles them to free emergency medical care anywhere in Europe, and a further five per cent think it will cover them worldwide.

Around seven per cent believe that, if they become seriously ill or are badly injured in Europe, having an EHIC means they will be flown home by air ambulance free of charge, and five per cent assume it means they don't need insurance.

In fact, although carrying an EHIC can make a significant difference to the cost of a medical emergency, the benefits are strictly limited - and it is no substitute for a full travel policy.

Caroline Lloyd, travel insurance spokesperson at Gocompare.com, said: "Having an EHIC can help you to access free or, more likely, discounted emergency medical services while abroad.

"But it's not a guarantee that you won't have to pay a thing, and it certainly won't be any help if you need medical repatriation to the UK, which can cost thousands of pounds.

"Having an EHIC may also mean you don't have to claim on your travel insurance and pay an excess for minor injuries. Some insurers will even waive the policy excess for medical claims where you've used your EHIC but where it doesn't cover the full cost of your treatment."

The EHIC entitles the holder to the same state medical care as would be given to an eligible person living in that country. It covers all 27 members of the European Union plus Switzerland, Iceland, Norway and Liechtenstein, but it isn't accepted in Turkey.

However, state care varies dramatically from country to country, and few EU members cover the full cost.

In France, for example, patients may have to pay for a consultation with a doctor, and although up to 70 per cent of the fee will be reimbursed later, they must bear the rest themselves. If a hospital stay is required, they may have to pay part of this too.

In Greece, treatment for a sprained ankle can cost £500, while in the US, which is outside the EHIC area, the bill for treating a broken leg could be £45,000 and £80,000 for a heart attack.

There are also no guarantees that someone requiring an ambulance abroad will be taken to a state hospital. Many smaller hospitals and clinics in holiday resorts are privately run and don't accept the EHIC, so there will be no contribution to the cost.

Medical repatriation can be hugely expensive too, and, in most circumstances, the UK authorities won't step in. Gocompare.com says one seriously ill holidaymaker brought home from the Canary Islands by air ambulance was forced to find nearly £23,000. A decent travel policy would have met this cost.

There are several other common holiday disasters the EHIC won't help with. Ms Lloyd said: "It won't cover your baggage or reimburse you in the event of a cancellation of all or part of your holiday, whereas most travel insurance policies will.

"Look at an EHIC as complementing your travel insurance rather than replacing it, or you could end up with a hefty medical bill."

Each person travelling must have their own card, and parents and guardians can apply on behalf of children in their care.

Although over half of those questioned by Gocompare.com thought they had to pay, including more than two-thirds of 18 to 24 year olds, the EHIC is free and the online application process is straightforward.

However, there are websites that will change up to £49 per card to "help" the unwary apply. To avoid being ripped off by these middlemen, go straight to the NHS site www.ehic.org.uk.

Don't leave it to the last minute to apply, as cards can take a couple of weeks to arrive. And if you already have one, check it is still valid as they must be renewed every five years.

If you don't yet have holiday insurance, don't buy from a travel agent or holiday company, as equivalent cover is almost always available more cheaply from comparison sites. If you go abroad more than once a year, consider getting annual cover instead of multiple single trip policies.

Choose a policy that provides at least £1m of medical cover - some offer £5m or more as standard - plus a minimum of £3,000 for cancellation, £1,500 for lost or damaged baggage and £250 of cash cover.

And be sure to take additional sports protection if you plan to try hazardous pursuits such as scuba diving or bungee jumping.