One in ten people travelling abroad this year may find they are not properly covered by their insurance policy, according to the AA.
The AA said travel agents do not ask enough detailed questions at the time cover is taken out.
AA Travel Insurance manager Andrea Crinnion said: "To provide cover, insurers need to know if there are any 'pre-existing medical conditions' - a term that can confuse many people.
"Quite apart from the millions who may have received some form of hospital treatment in the last year or who have other conditions that should be declared, at least a tenth of the population suffer from serious but controllable complaints such as angina, asthma or diabetes.
"These people may overlook their condition because they live with it year after year - it's a part of their life."