French car hire firm Europcar has admitted that it may have to pay out as much as £30m to British motorists who were overcharged for car repairs.
UK Trading Standards officers launched an investigation after its office in Leicester received complaints.
The Serious Fraud Office (SFO) is also planning to launch an inquiry, according to the Daily Telegraph.
The paper says more than half a million motorists could have been overcharged for repairs over many years.
In a statement, Europcar said: "Europcar's view is that the implications of the investigation will be somewhere in the region of £30m."
But the company - whose shares fell by 2.5% on Monday - said it had no further comment to make.
The Telegraph said some people were charged four times what they should have been for routine repairs.
The figures suggest an average compensation payment of up to £60 for every motorist who was overcharged.
Europcar's website says it charges an administration fee of £40 for each repair, plus up to £25 for a replacement wiper blade, and up to £350 for replacing a tyre.