Complaints about car hire in the EU rose by almost 30% last year, figures show.
Post-hire damage charges, particularly after unsupervised drop-offs, disputes over insurance and fuel policies, were the top causes for complaints from UK consumers, according to the UK European Consumer Centre (UK ECC).
There were 629 complaints to the centre from July last year to June this year, up from 486 the year before - an increase of 29.4%.
Car hire companies in Spain, Italy, Iceland and Malta caused the most problems for UK travellers.
The UK ECC said a typical complaint resulted from a consumer receiving a bill for damage from the car rental firm up to months after returning the vehicle.
Other complaints included problems with insurance, such as overpriced or unrequested excess waiver charges, and fuel policies such as charging to refill the tank even when it was not empty.
UK ECC director Andy Allen said: "Consumers are often at their most vulnerable during tough economic times and, quite naturally, they often have their guard down and are more trusting when they are on holiday.
"Like any transaction, car hire has risks associated with it, but in the light of these figures showing increases in complaints about car rental, we would urge consumers to be aware of some of the problems they could face and to take steps to make the whole process go as smoothly as possible.
"Our general advice to consumers hiring cars abroad is, where possible, to return the car to someone in the office, get them to check the car and sign it off as in good condition. If you have to leave the car, take some photos showing that it was returned in good condition."