Airport stores have been accused of not being straight with their customers as it is alleged some demand to see boarding passes only so they can avoid paying tax on their products.
Items which are being taken to destinations outside the EU are exempt from VAT, and retailers can avoid the tax if they produce evidence such as a scan of a boarding pass.
Retailers are not obliged to reduce the prices of zero-rated items, meaning savings may not always be passed on to customers.
Passengers travelling outside Europe should benefit from the savings companies can make from the tax exemption, consumer affairs expert, Paul Lewis told the Independent.
"I think the problem here is that the retailers are not being straight with the public," he told the newspaper.
"They are asking to see passengers' boarding cards but not telling them that this is so they can make more money by not paying the VAT on what they're selling.
"What of course they should be doing is passing on the savings that they make to the passengers who are travelling outside Europe.
"The problem is, though, that they have got a captive audience."
An HMRC spokeswoman said: "Airport tax or duty free shops may treat the sale of goods to passengers intending to take them to non-EU destinations as zero-rated exports, provided they retain suitable evidence such as by scanning the boarding card.
"There is nothing in VAT law to require the production of a boarding pass to purchase goods in airport shops, but without such evidence the supply cannot be zero-rated as an export.
"HMRC cannot comment on the commercial pricing policies of individual retailers nor about when their staff ask to see boarding passes."