In research published this week, Which? revealed nearly half (47%) of Interrent customers and more than four in ten (42%) Goldcar customers reported a problem with their rental.
The next worst offenders were Thrifty (31%), Dollar (29%) and Firefly (29%).
Larger rental brands fared better, with 18% of Sixt customers reporting problems, 21% of Enterprise customers, 24% of Europcar customers, and 27% of both Avis and Hertz customers encountering issues.
“We get more complaints from Which? members about aggressive sales, unauthorised and unexpected charges, and appalling customer service in this industry than any other travel-related service,” says the report.
The organisation is launching a more extensive investigation into three key issues, focussing on high insurance excesses, unauthorised and unfair charges, and questionable damage repairs.