The consumer organisation used readability software to analyse 40 policy documents from the 10 largest car, home, pet and travel insurers. It examined the length of words and sentences to estimate how well-educated a reader needs to be to understand the language used in a text.

The tests revealed the average document was more unwieldy to read than Stephen Hawking’s A Brief History of Time and Dostoevsky’s Crime and Punishment – a level that could cause problems for more than two in five (43 per cent) working adults with a reading ability of GCSE grade C or below.

A Brief History of Time and Crime and Punishment require secondary or sixth form-level reading abilities. Although readability tests do not take into account the subject matter, so the concepts in A Brief History of Time may be less digestible than the subject matter of an insurance policy document.

In a separate snapshot investigation, Which? put six home and travel policy documents to the test to find out if they were practical to use.

Twenty-four participants were asked to read two policy documents each and answer a number of questions based on different scenarios, such as how to make a claim or reporting a change in circumstances.

On average, participants answered five out of 16 of questions incorrectly when reviewing the travel insurance policy documents. For home insurance, the average was three out of 12.

Even a retired insurance professional, civil servants and software engineers were unable to answer all the questions correctly in our test. One participant said they felt a document they tested was written for “lawyers and solicitors” not for “everyday people”.

Which? tested policies from insurers and found participants answered a third of questions relating to Santander’s travel insurance policy incorrectly and approximately one in four for Axa and Insure & Go.

With home insurance, nearly a third of questions for Direct Line were answered incorrectly, one in four for Aviva and one in five for the Post Office.

Ceri Stanaway, Which? money editor, said: “Millions of insurance policies are bought every year, so it is worrying the policy documents are often far too complex for the average customer to understand, as our investigation suggests.

“Unclear insurance policies can have devastating consequences for customers, who could see their cover invalidated due to a misunderstanding.

“Customers can use their insurer’s glossary to make sense of complex terms, and should give their insurer a call if something is unclear, however, we want to see all insurance providers taking steps to cut out the jargon and make their policy documents easy for customers to get to grips with.”