Broker AA Insurance Services has hit out at its rivals over the lack of information requested by supermarkets and travel agents selling travel insurance.
In its Summer report, the AA claims one in ten people travelling abroad this year may find they are not properly covered.
It also attacked travel agents for not asking enough detailed questions at the time cover is taken out.
Andrea Crinnion, manager of AA Travel Insurance said: "To provide cover, insurers need to know if there are any 'preexisting 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 easily overlook their condition simply because they live with it year after year - it's a part of their life."
