Easter holidaymakers should now be safer from hidden charges when hiring a car, competition authorities say.
Action was prompted by a review of the car rental industry which found many firms were advertising low prices by hiding key extra costs.
Some 30 leading UK websites have now promised to show clear pricing, including compulsory costs in quotes.
Another 20 websites, accounting for 1% of the market, have been sent warning letters.
Consumer protection law requires transparent pricing, which includes all compulsory costs.
The UK's Competition and Markets Authority (CMA) has overseen various improvements ensuring that charges such as fuel surcharges, young driver fees, and out-of-hours pick up charges are made clear.
The CMA claimed it had "vastly improved" the sector. Now it said it was taking enforcement action against car hire companies based overseas but selling directly to UK customers online.
"This move sends a clear message: just because a business is not on UK soil doesn't mean the law doesn't apply when it sells in the UK," said Michael Grenfell, director for enforcement at the CMA.
The announcement comes as data collected by the UK European Consumer Centre between March 2017 and February 2018 showed a 36% increase in complaints and inquiries from UK consumers compared with the previous year.
Complaints have included hidden charges for fuel, high insurance excess, and other hidden insurance costs.
Leon Livermore, of the Chartered Trading Standards Institute, said: "UK consumers have enough to contend with without worrying about hidden charges for car services. The CMA guidance and action will ensure businesses are transparent and upfront so that consumers can make an informed decision."