British and Spanish consumer watchdogs have called on car hire companies to end unexpected charges and unfair fuel policies after a poll found almost three quarters of tourists believed they had been "ripped off".
Which? said many tourists to Spain were getting a raw deal, and the Spanish group Organisation of Consumers and Users (OCU) said unlawful practices by car hire companies were damaging the image of the country as a holiday destination.
Which? said confusing policies meant holidaymakers were often hit with additional, non-refundable charges for fuel that were not always clear at the time of booking or buried in the small print.
It cited the full-empty policy where hirers must pay for a full tank of petrol on arrival but are not offered refunds for returning the car with unused fuel.
Almost three quarters of Which? members (72%) who had experienced this thought they had been "ripped off" by being forced to pay a fuel price that was higher than at local garages.
Some six in 10 members who booked a car with a full-empty policy said it was the only option available to them.
The watchdog said tourists could find there was little chance of using a full tank, especially on the small Spanish islands, and estimated that someone hiring a Fiat 500 would have to drive the 80 mile round trip from Palma airport to Port de Pollenca on the other side of Mallorca nearly seven times to get their money's worth.
Spain is the most popular foreign destination for British holidaymakers, and a fifth of Which? members (22%) who hired a car on holiday did so there, making it the second most popular car hire destination behind the UK.
Which? found a number of companies did offer a "fair" fuel policy, such as Alamo, Enterprise and Autoreisen.
Which? executive director Richard Lloyd said: "There can be few things guaranteed to ruin your holiday quicker than finding surprise charges and fuel costs unfairly added to your car hire bill.
"We want an end to this holiday rip-off, with car hire companies totally up-front about all charges, so people know exactly what they will pay when they book."
OCU spokeswoman Ileana Izverniceanu said: "We hope that Spanish authorities investigate and punish all rental car companies which engage in unlawful practices.
"This behaviour damages consumers' rights and the image of Spain as a holiday destination."
Which? said it and OCU would be asking Spanish consumer authorities to take action to protect all consumers from unfair fuel policies and had already shared its concerns with the Competition and Markets Authority in the UK.
Which? surveyed 2,246 members online in November last year.