They could find themselves driving away with a vehicle with dodgy tyres, sloppy servicing, faulty headlights or missing indicators, according to a survey carried out for Holiday Which? magazine
Of 57 cars inspected in Corfu and Spain last autumn, 17 were condemned as dangerous.
The inspectors estimated that the chances of hiring a car that a customer would be happy with were less than 50-50.
Only five of the 57 cars passed the inspections with no faults, while another 15 passed muster but with minor, non-safety related problems.
One in three cars were either badly serviced or not adequately checked between hirers.
Dangerous tyres were the most common problem, with 16 out of 57 cars falling into this category.
The risk of a blow-out is much greater in a tyre with a bulge in the side wall or a cut through to the fabric. And road-handling is seriously affected by tyres that are bald or with tread less than 1.6mm deep, especially in wet conditions or where dust or sand has been allowed to accumulate on the road surface.
Among other cars tested, they found headlights and brake lights not working, seriously irregular tyre pressures and a car that overheated only 5 kilometres from the hire shop.
One of the worst examples was a Fiat Panda hired in Corfu town which had sharp edges sticking out of a bashed-in wing, an indicator missing, and a bent wheel (indicating possible steering damage).
"A bald, cut and over-inflated tyre, a faulty headlight and dodgy spare tyre made this car even more of a deathtrap," the inspectors commented.
The magazine advises holidaymakers to choose a company with a British base and to look at their hire car very carefully, particularly the tyres before driving away.
"We are very worried that serious faults are not being repaired - or even spotted," said Patricia Yates, editor of Holiday Which?
"People's lives are being put at risk. We have asked hire companies to take these unfit cars off the road, and take more responsibility for the safety of their customers.Reuse content