Which? reported that this practice was on the increase in the industry as a whole, but the highest number of complaints were from Alicante and Malaga.
One of its researchers found they would need to drive nearly 140km a day to use up their tank of petrol.
Chris Gray of Which? Travel said: "No-frills airlines have their hidden card surcharges. In car hire, the hidden charge has become fuel. Consumers should not be in a position where booking a car rental commits them to paying an unavoidable charge with no refunds, with no idea how much the charge may be."