The car hire industry is being investigated following allegations of fraud.
Europcar has disclosed that consumers may have been overcharged by a staggering £30 million.
A newspaper investigation found more than 500,000 people may have been charged for 'fraudulent' repair bills that were inflated by up to 300 per cent.
The Daily Mail reports that some consumers who have damaged a hire car while abroad have paid between £1,000 and £2,200 for overcharged repairs.
It is also alleged that rental firms use these charges to persuade drivers to pay for better insurance.
Alex Neill, from Which? said: "If Europcar is found to have inflated costs for repairs, people will be outraged.
"There should be an immediate investigation to ensure others aren't doing this and customers who have been left out of pocket are compensated without delay."
The Sun also reports that British hire car customers traveling in Europe have also been hit with a 'mysterious' charge since the Brexit vote.
A Europcar spokesman said: "Following the recent commencement of an investigation, last June 23, by UK Trading Standards into repair costs levied by Europcar UK, the company is undertaking a thorough investigation and Europcar UK is fully cooperating with the authorities.
"Given the ongoing investigation Europcar cannot provide any more substantive comment at this stage."