Norwich UNION has declared war on cowboy builders after finding that three million homeowners have fallen foul of rogue tradesmen.
The insurer hopes its Botch Watch campaign will highlight the issue for consumers ahead of a government clampdown planned next year.
The shocking figure that one in five homeowners has been the victim of slipshod repairs comes from an independent survey commissioned by the insurer.
The survey of 500 homeowners also revealed that 73% of people feel they lack rights when complaining about the standard of builders' work. Only one in three people bothered to check a tradesman's references before allowing them to start work.
Norwich Union found that a quarter of all homeowners have had major construction work carried out on their homes in the past three years.
Neil Phillips, Norwich Union spokesman, said: "Cowboy builders or repairers pose a real financial risk to consumers. They can cause emotional stress and put people's safety at risk."
However, the survey discovered that many victims are too embarrassed to complain to builders about work with which they are not satisfied so there are no reliable figures on the cost of the problem.
To combat this risk, Norwich Union has published a guide on the pitfalls and perils of selecting builders, called How to Avoid Hiring the Bad Guys.
More than 8,000 brokers and intermediaries are being sent copies of the guide for them to hand on to policyholders. It includes contact details for trade associations that are able to recommend reliable builders and tradesmen as well as handy hints on planning repair work.
David Sibbert, chairman of the Trading Standards Institute, said: "The independent study commissioned by Norwich Union provides national evidence of the need for urgent action to protect consumers from unacceptable behaviour by cowboy builders."