Flight Centre, one of the UK's leading travel agents, has been accused of cheating holidaymakers.
A new investigation by Channel 4 programme Dispatches shows staff being filmed discussing tricks used to mark up prices.
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The programme shows investigator Harry Wallop and his 81-year-old mother-in-law going to different branches of the high street chain asking about flights to New York on the same day.
Mr Wallop revealed he'd checked prices online and was quoted £494, while his mother-in-law, who had not, was quoted £610.
The Mirror reports that Flight Centre dismissed the differing prices quoted for the same flight as "extraordinary" and "one isolated example".
It added it was "not unusual or unethical to apply a margin" and insisted their prices "honoured their Lowest Airfare Guarantee".
The Daily Mail reports that other tactics of adding hidden mark-ups to tickets include 'seat-blocking', where Flight Centre staff allegedly reserve cheap seats to make it look like only more expensive ones are available.
"We are surprised to see from Dispatches' press release that they appear to have ignored the evidence we have sent them.
"Dispatches' allegation that Flight Centre has a policy or culture of applying opportunistic or excessive margins to particular customers or demographics is completely unfounded.
"To set the record straight, our shops' average margin on long haul flights is: 7.9% for customers aged 26-35 and 7.8% for customers aged 66+.
"On an average airfare of £721, this works out to a difference of only £0.72.
"Our consistently low margins and access to discounted flight deals keep us competitive. We are proud that our Lowest Airfare Guarantee enables our customers to secure a better price than that available from our competitors.
"We acknowledge that this programme – which we have not seen in advance – may highlight some isolated behaviour that is against our company policies and ethics.
"We are taking this very seriously and will deal with it immediately to ensure that all our customers enjoy the same high levels of service and value that the overwhelming majority do today."
Flight Centre is one of the world's biggest travel agents and is worth £250 million a year in the UK alone.
Dispatches: The Truth About Cheap Flights airs at 8pm on Channel 4 on Monday 25 April.