A man says he was left "horrified" when a car dealer sold him a car without telling him it had been previously written off.
Channel 4's Dispatches said the practice is increasingly common as even some of the biggest dealerships "are advertising as pristine cars that have been in serious accidents".
These second-hand cars are bought up, repaired and sold on to buyers - some of who are never told about the original accidents.
In some shocking cases cars are even sold with no airbags - after they have been simply cut away following a previous accident.
One man, referred to just as David, bought a second-hand Mini last year.
The car looked to be in good condition.
Then, eight months later he tried to part-exchange the car for a newer electric vehicle.
He told Dispatches he then realised the car had been involved in an accident that he was never told about.
When he saw photos of the crashed vehicle, David said he had the shock of his life.
"In viewing the images I was absolutely horrified," he said.
The photos showed the car had been in a crash so bad that it had lost a wheel, had sill damage and had multiple airbags deployed.
David said: "Not only was it an insurance write-off, it looked to me to be in a condition beyond repair.
"But more than that, the fact that I put my life in that car, to drive around in for eight months.
"I struggle to understand how that could happen."
With the aid of a claims firm, David managed to get the dealer to take the car back and give him a refund.
But then the car was simply listed for sale again - at the same price David had bought it for.
Dispatches said: "If someone knowingly sells a car without declaring it is a write-off, it breaks the law."
Claims specialist Ian Ferguson estimates that around 40,000 undeclared write-off cars are sold like this each year and those sales could be worth around £800million.
In the programme, Ferguson said: "What I’m seeing is, in the cases I deal with, the provenance checks through the process of the buying , are not happening.
"Which means that more and more of these uncategorised write-offs are finding their way through from auction sites which are serving the insurance industry and then they find their way into a used car seller.”
He added: "There’s more sinister undertones to these cars as well insofar as we have cars with airbags that have gone off and instead of replacing the airbag, which would’ve been many, many thousands of pounds, the airbags [have] simply been cut away and the systems been reset, the cars been repaired and repainted and put back out in the retail food chain.
"That could have fatal consequences if someone was in an accident.”
The Dispatches programme 'Why Is My Car So Expensive?' is on tonight at 8pm on Channel 4 and All 4k.
