Credit card giant Mastercard is to raise the fees it charges EU merchants when UK cardholders buy goods and services from them online by fivefold.
It has sparked fears that consumer prices could rise if merchants choose to pass on those costs, especially on items not available from UK retailers.
Transactions with airlines, hotels, car rentals and holiday firms based in the EU could all be affected.
Mastercard attributed the move to the UK's decision to leave the EU.
It said that only online sales would be affected and that "in practice" UK consumers would not notice the change.
The change affects the "interchange" fees Mastercard sets on behalf of big banks, so that its customers can use their payment networks.
From October, Mastercard said it would increase these fees to 1.5% on every transaction, up from 0.3%.
The EU introduced a cap on such fees in 2015 after concerns they pushed prices up for consumers and unfairly burdened companies.
British customers makes tens of billions of pounds of purchases every year from European merchants on credit cards alone - and the hike in fees from Mastercard will affect the majority of those.
The increase may be relatively small but it's significant, coming at a time when retailers may face extra paperwork and checks - higher costs - for goods coming into the UK.
With Covid restrictions bringing their own challenges, businesses, especially smaller ones, may be compelled to pass on the costs to consumers.
And it's not just items crossing borders. The payments for most items bought on Amazon in the UK are processed via its Luxembourg headquarters.
With the increase not coming in for several months, international companies may look at ways to reclassify UK sales, to avoid the charges.
Mastercard is implementing the rises simply as it's no longer bound by the restrictions imposed by the UK being in the EU. The banks which receive the fees have said in the past that they are invested in areas such as card security and innovation. This time, however, the trade body which represents them has declined to comment on the rises.
But Mastercard said that since the end of the Brexit transition period, the cap no longer applied to many payments between the UK and European Economic Area (which also includes Iceland, Liechtenstein and Norway).
"As a result of the UK leaving the EEA, Mastercard will adapt interchange rates on UK cards to the commitments it gave the European Commission in 2019 for non-EEA card transactions," the company said.
"In practice, only EEA merchants making e-commerce sales to UK cardholders will see a change."
Kevin Hollinrake, chair of the parliamentary group on Fair Business Banking, told the Financial Times, which first reported the story, that the move "smacks of opportunism".
And Callum Godwin, chief economist at CMSPI, the global payments consultancy, said airlines, hotels, car rentals and travel groups would be hit.
"[This will happen] anywhere the consumer is in the UK and the merchant is in the EU," he said.
He added that many firms in these industries were already struggling due to the pandemic.
Visa, Mastercard's larger rival, has not announced plans to change its fees but told the FT it was keeping the issue under review.
Companies in the UK and EU are already facing added costs and delays due to post-Brexit trade rules brought in on 1 January.
Some EU exporters have already stopped deliveries to the UK because of new VAT related charges.
Meanwhile, UK consumers who have bought goods from firms based in the bloc have found themselves facing hefty charges to cover customs duties, taxes and administration.