Hidden fuel charges have caused holiday car hire prices to triple, an investigation by Which? Travel has found.

Prices quoted in the UK increased up to three-fold after the cost of hire-cars and fuel were added.

The cheapest initial car-hire price in Malaga, including basic insurance, was €30.25 from Goldcar, but when fuel costs were added the price rose to €94.25, the investigation found.

In Alicante, Which? Travel researchers rented a car from Holiday Autos for €28, but when the fuel was added it came out at €144.77, including enhanced insurance.

Companies used a ‘full-empty’ policy, which charged customers for a full tank of petrol when they pick the car up and they are not refunded for the fuel they do not use. This is not made clear to customers when they make the booking.

Rochelle Turner, an expert for Which? Travel, said: “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.”

A spokesperson for Hertz car hire, said: “Regarding fuel charges, Hertz customers can choose the Fuel Tank Purchase Option (FPO), this means customers can purchase a tank of fuel from Hertz at the time of rental, and is indicated in their rental agreement.

“If Hertz’s Fuel Purchase Option is not purchased in advance, we charge the customer for fuel to fill the tank when the vehicle is returned; for example, if the car is returned with three-quarters of a tank of gas, Hertz charges them for one quarter of a tank. This is indicated at the time of booking.”