Hidden fuel charges are bumping up the cost of car rental abroad by up to three times, an investigation by Which? Travel has found.

Which? Travel sent undercover researchers to Spain to investigate car hire companies, after receiving complaints from Which? members about being hit by large charges for fuel that were not made clear at the time of booking.

We found prices quoted in the UK increased up to three-fold once we picked up the hire-car and the cost of fuel was added.

Our researcher in Malaga was told that the cheapest initial car hire price including basic insurance was from Goldcar at €30.25. But when fuel was added on arrival, the price rose to a whopping €94.25.

In Alicante we rented from UK broker Holiday Autos, locally-based company Centauro and direct from Hertz. At the time of the booking, Holiday Autos was the cheapest at €28, compared to €52.70 for Centauro and €65.58 for Hertz. However, once we picked up the car and fuel was added on, the Holiday Autos price rose to a huge €144.77 including enhanced insurance and fuel.

Companies used a ‘full-empty’ policy, in which consumers are charged for a full tank of petrol when they pick up the car and given no refunds for any fuel they do not use. The impact of these charges on the total price is not made clear when customers made the booking.

Which? Travel expert, Rochelle Turner, says:

“It’s clear from our investigation that consumers are not being given the full facts about these extra charges. Our research has shown that these unavoidable fuel charges can triple the price of a rental, turning what seemed like a good deal to potentially a very expensive one.

“All charges should be shown up-front at the time the booking is made so consumers can make the right choice and compare prices easily.”