Customers of budget airline bmibaby may have been over-charged up to £60 because of hand luggage measuring discrepancies.
People flying on the airline can take bags on board for free if they are no bigger than a certain size.
But an investigation by the BBC's Watchdog programme - to be shown tonight - has shown that gauges at bmibaby departure gates in at least three airports are too small for bags of the permitted size to fit inside.
Now bmibaby is to replace a number of measuring gauges.
One of the airports where Watchdog found a faulty gauge was at East Midlands - bmibaby's main base.
On tonight's programme passenger Kirin Somaiya tells of his experience after booking a return flight to Amsterdam earlier this year.
"I was told to measure my small suitcase and I was told to put it into the cage to see if it fit," said Mr Somaiya, a 29-year-old businessman from Leicester.
"So what I did was try to pop it into the cage that they had there at East Midlands Airport, and it simply didn't fit in - it was just too big and too wide for it to slot into the cage."
Having already bought his return tickets, Mr Somaiya felt he had no option but to pay for his bag to go into the plane hold.
When he got to Amsterdam's Schiphol Airport and found his bag fitted into the bmibaby gauges there.
In response to the programme's report, bmibaby said that some of its gauges had been designed with curved edges, meaning that a box that met the maximum luggage dimensions would not be able to fit inside.