Brits are being warned to declare this one thing on their travel insurance or risk bills costing more than £150,000. Finding travel insurance is an important part of planning a holiday, providing financial protection if you run into any problems while away.

It can be used to cover unexpected hospital bills, lost or stolen items, trip cancellations and even emergency transport. Without the insurance these can be incredibly expensive when in another country.

However, when applying for cover, failing to disclose any previous medical issues could completely invalidate your insurance. The Foreign Commonwealth and Development Office (FCDO) warned: “If you travel internationally you should buy appropriate travel insurance before you go, covering you for existing physical or mental health conditions (including those currently under investigation) and any activities you will be doing whilst you are away.

“If you do not have appropriate insurance before you travel, you could be liable for emergency expenses, including medical treatment, which may cost thousands of pounds.”

More specifically, it explained: “Declare existing conditions or pending treatment or tests so that you are covered if there are related complications during your trip; failing to declare something may invalidate your travel insurance.”

The FCDO shared examples of how much this could ultimately cost you if it voids your insurance:

• Fall and break your leg in Spain, you will need hospital treatment and possibly medical evacuation/repatriation – £25,000+
• Quad bike accident in Greece, and you need surgery and medical evacuation/repatriation – £80,000+
• Stomach bug or infection treated in a hospital in the USA and possibly medical evacuation/repatriation – £150,000+.

But what counts as a pre-existing condition. Money Saving Expert (MSE), the site founded by Martin Lewis, says that some insurers may need details of conditions taking back many years.

It says: “It’s important to carefully read the questions asked by the insurer – they may want to know of medical conditions or visits for medical attention within the last three, four or five years, and some insurers may even want to know your medical history even further back, as you’re usually considered to be more likely to make a claim.”

The site lists some of the most common medical conditions that you need to disclose when buying travel insurance – though this is not a complete list:

• Epilepsy
• Heart conditions (including high blood pressure or cholesterol)
• Asthma
• Diabetes
• Mental health conditions (including depression, eating disorders, anxiety)
• Arthritis
• Gout
• Crohn’s disease.

It is also worth knowing that you’ll also need to share details of:

• Treated broken limbs
• Any surgery
• Any ongoing treatment (including if you’re waiting for an operation or investigation)
• Any condition where you’re awaiting test results.

MSE adds: “It may sound like a daunting task but it will help you to find the right policy for you. If you do not share this information with the insurer, it is very likely to invalidate your policy and any cover.”

Price comparison website Money Supermarket adds that travel insurance if you have a pre-existing condition may be more expensive. It says: “Yes, you can get travel insurance if you have a pre-existing medical condition, although it might be more expensive than standard travel insurance. This is because an insurer sees you as a higher risk of making a claim on the policy.

“However, there are still lots of policies to choose from for those with pre-existing conditions. They cover a wide range of health problems and there are also specialist insurers who can help.”

Before travelling, the FCDO recommends you take your insurance policy details with you, including the policy number and your insurer’s emergency assistance telephone number. “Share your policy details with people you’re travelling with and friends or family at home, in case they need to contact your insurance company on your behalf,” it adds.

Alongside any pre-existing conditions you should also double-check whether your policy covers:

• The full length of your trip (many policies have a maximum trip length and/or an annual limit on how much time in total you can spend outside the UK)
• Treatment in state or private hospitals (emergency treatment and hospital bills can be enormously expensive)
• Emergency transport, such as an ambulance: this is often charged separately to other medical expenses and emergency travel home on medical grounds can be very expensive
• All activities you may undertake on holiday, such as sports or adventure tourism (you may need specialist insurance or an add-on for some activities)
• All the places you intend to visit, even if only in transit, in case anyone needs emergency treatment in a transit country
• Repatriation costs if you or a family member dies abroad
• Getting home after medical treatment if you cannot use your original ticket
• Reasonable costs for a family member or friend to stay with you, or travel out to accompany you home if required.