Mobile network EE is to charge customers for using their phone in Europe from next year.
The company, owned by BT, had previously said it had no plans to reintroduce the charges after Brexit.
It will affect new customers and those upgrading from 7 July.
They will face a £2 daily fee from January next year to be able to use their data, call minutes and text allowances in 47 European destinations.
The change will not apply to customers travelling to the Republic of Ireland.
British travellers have not had to pay roaming charges on their mobile phone bills since June 2017, when they were abolished after changes to European regulation.
Before then they added an estimated £350m a year to users’ bills.
Britain’s departure from the European Union meant that from January UK customers no longer had the right to use their phone in Europe without roaming charges.
However, Britain’s major mobile operators had said they had no plans to introduce them.
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EE said its decision was designed to “support investment into our UK-based customer service and leading UK network”.
It said customers travelling abroad for longer will be able to use a 30-day Roam Abroad Pass.
Ernest Doku, a mobiles expert at Uswitch.com, said: “It’s hugely disappointing for consumers to see that situation change so quickly.
“If you’re an existing EE customer, these charges won’t affect you yet, but make sure you check the small print if you’re due an upgrade in the coming months.”
Sky News contacted other networks to ask if they had any similar plans.
O2 and Three said they had not made any changes to roaming.
However, both are altering their policies on “fair usage” daily data limits while in the EU – though in each case said the limits were more than enough for the vast majority of holidaymakers’ needs.
Vodafone has also been contacted for comment.