According to the consumer organisation Which? nearly a third of people who told their travel insurer about a new medical condition, that had developed after the policy was purchased, had to pay a higher premium.
Many insurers include in their travel policies a get-out clause (called an ongoing medical warranty) that covers new medical conditions which allows them to increase premiums or even deny cover altogether, even if the customer has been given the all clear by a doctor to travel.
This is still happening despite the Financial Ombudsman Service (FOS) ruling several years ago that it was generally not fair for insurers to refuse cover on new medical conditions that arose between policy purchase and date of travel. The FOS ruled that this would only be fair if the new condition was so major that the risk insured became completely different.
Not all insurers include these medical condition get-out clauses, so make sure, as always, to check the small print before purchasing any form of insurance. According to Which? research policies from Axa Insurance, Freedom Travel Insurance and Miaonline.co.uk do not include such get-out clauses.