Car hire firms are charging customers more than double the going rate for repairs.
An investigation found that customers were getting charged up to £1,000 extra.
Which? Travel collected evidence of damage from car hire customers across Europe who believed that they had been overcharged.
In 75 per cent (eight out of 12) of cases that were investigated, alternative garages quoted less than the car hire companies to do the work.
In four of the cases, the car hire company charged double the average cost suggested by alternatives.
Of the 36 quotes received from the Which? Trusted Trader garages, only eight were equal to or higher than the rental firm’s charge.
One of the cases found that a person who was renting a vehicle in France was charged £1,154 to repair a windscreen chip.
However, that this chip could be fixed for as little as £35 with a different garage.
At best there was a markup of 300 per cent added to the repair cost but sometimes the bill was 30 times the cost of the repair.
Some drivers said they suspected car hire firms may have been charging multiple customers for the same minor damage to a vehicle.
Many major car hire firms – including Avis, Budget and Hertz – admitted to Which? that they often do not carry out repairs, even when customers pay for them.