Sainsbury Finance calculates that up to 24 million policy holders have guessed when supplying important information. The insurer also estimates that over a million people have lied on their policies.

Steve Johnson, head of insurance at Sainsbury's Finance, said: "If unsure of the answer to a question when getting a quote or buying cover we would encourage people to always take the time to go back and check any details.

"In some cases you can retrieve online quotes and modify them later."

Home insurance policies receive the most inaccurate details with 18 per cent being incorrect. This is followed by car insurance policies of which 15 per cent have the wrong information and travel insurance policies at 12 per cent.

Mr Johnson added: "It really is quite concerning that so many people seem to think it doesn't matter if they take a guess at the information requested at the point of taking out cover."