The Office of Fair Trading should look into car park payment machines that do not give change, the Government has been told.
Liberal Democrat Lord Teverson said hospitals and local authorities raised £1.25 billion from parking charges.
He told peers at question time: "Ten per cent overpayment means perhaps £100 million wrongly collected. There is all sorts of new technology, so isn't this exactly where the OFT should get involved?
"Where else can traders take your money and get away with refusing to give you change?"
Lord Bach, for the Government, said the choice of equipment for collecting parking charges was a commercial decision and "thankfully not a matter for central Government".
But Labour's Lord Borrie, a former Director General of Fair Trading, hit back: "In many situations the car park operator is a monopolist.
"In the hospital situation it may be miles and miles away from the hospital before you can find another possible car park.
"If there is a monopoly situation surely any form of exploitation is something the minister should be concerned with?"
Lord Bach said it was not that he was unconcerned, but that the matter had been devolved in 1986.
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