Many of the top car hire companies are failing to disclose extra costs, making it almost impossible for consumers to find the best deals, a watchdog says.

Which? attempted to rent a car in Spain from 10 major hire firms and broker websites, but by the end of the booking process 53% of its researchers were not satisfied they knew the total price. It found 75% of those who had to pay a compulsory cost for a full tank of fuel did not know how much they would need to pay when they collected the car.

More than one in five (22%) did not know the amount of the excess insurance cost and 44% did not know the price of the optional excess waiver to reduce the amount they would have to pay if the car was damaged. Such payments can add more than £100 to the cost of a week's rental.

Some 54% did not know if the excess waiver covered damage to tyres, windscreen or under the vehicle.

Which? wants all fees to be made clear to consumers upfront and before they make a booking, saying it is a principle of European and UK law that information such as compulsory and other significant charges must be clearly stated at the point of purchase.

Smaller companies topped a customer satisfaction survey of car hire firms by the consumer group, with Canary Islands-based Auto Reisen beating more well-known names with a score of 89%. Goldcar was bottom of the table with a customer score of 42%.

Thrifty and Enterprise took second and third place respectively, while Europcar and Budget rounded out the bottom three.

Economy Car Hire topped the broker table of ratings with 78%, with Argus Car Hire at the bottom scoring 52%. Which? executive director Richard Lloyd said: "The car hire industry is taking customers for a ride by hitting them with sneaky charges not included in the headline price.

"Not being upfront about the total cost makes it almost impossible to shop around for the best deal and leaves people on tight budgets paying more than they planned."