The Sunday Times has learnt that up to 20 per cent of British travellers are being overcharged when renting a car.

The rental companies say that such incidents are rare, while travel companies, which enjoy close relationships with car-hire firms, have tended to keep quiet on the subject. But now one tour operator has broken ranks to support our campaign, claiming that overcharging is rife within the industry, and that one in five of its clients is being overcharged.

The company’s managing director, who has asked to remain anonymous, says:

“We have had an explosion of complaints where clients have been wrongly charged for refuelling, late deliveries and damage claims. In every case, my clients have been proved innocent.” The travel operator, who works mostly in Spain, says that until recently about one in 10 of his clients complained about extra charges for car hire. “But we have received a spate (of complaints) taking that to about 20%,” he says.

“In all cases, we successfully claim the money back for the client, but it’s a real pain for us, and I do wonder about the poor people who book direct and do not have the contacts to get refunds.” The MD finally lost his patience when he was himself overcharged after a recent visit to Spain.

Holidaymakers who book hire cars through tour operators can take their complaints back to the operator, but the tens of thousands who book direct are on their own when it comes to trying to extract refunds.

Fortunately, individuals can get results. Our postbag reveals that many Sunday Times readers are using advice published in the Travel section to lodge successful complaints with customer-services departments in the UK (dealing with offices abroad appears to be a fruitless exercise).

Stuart Simpson of Devon rented a car to get from Stansted to Newquay after Ryanair cancelled his flight home. He returned the vehicle, as arranged, to an unmanned National office at Newquay airport. The company later billed him £260, claiming he had left the car with a scratch and a dent. Incensed, Simpson complained to National, citing the Sunday Times campaign. Customer services agreed that the damage was “pre-existing” and offered him a full refund.

David Solomon of St Albans returned a car to Charles de Gaulle airport, Paris, and asked Budget staff to inspect it to confirm that it was undamaged. They declined to do this, but he was subsequently presented with a bill for £306. The company said he had damaged the rear bumper.

After complaining to customer services, Solomon received a full refund and the offer of a £35 voucher. He returned the voucher and demanded compensation for his time and personal distress. Budget at first refused, but eventually paid £120 as a “goodwill gesture”.