Customers of O2 have been told they will be billed £3.50 for every gigabyte (GB) of data used above a new limit of 25GB, from August.
The move comes after the Christmas Eve trade agreement signed by the UK left open the option of the return of roaming charges – which were scrapped across the EU in 2017.
In an embarrassment to the prime minister, it was announced on the fifth anniversary of the Brexit ‘yes’ vote, as he hailed the result as a spur to improving people’s lives.
“The decision to leave the EU may now be part of our history, but our clear mission is to utilise the freedoms it brings to shape a better future for our people,” Mr Johnson told MPs.
At Christmas, all the biggest UK operators said they had no plans to reintroduce roaming charges, but ministers admitted they had no plans to prevent them doing so.
The agreement merely encouraged operators to have “transparent and reasonable rates” – raising the risk of a return to the huge pre-2017 bills, sometimes running to hundreds of pounds.
In its email to customers, O2 announced “changes to our roaming fair usage policy”, from 2 August.
“As your monthly UK data allowance is over 25GB, you can still use your data in our Europe Zone,” one customer was told.
“But it’s now subject to a Roaming Limit of 25GB. Once you’ve reached this limit you’ll be charged an additional cost of £3.50/GB.”
The Independent has attempted to contact O2 about the switch, but nobody at the company answered the phone.
In December, O2 played down the return of roaming charges, saying: “We’re committed to providing our customers with great connectivity and value when they travel outside the UK.”
The government says it has passed legislation to protect holidaymakers, including a £45-a-month limit on any charges for using mobile data abroad before having to opt into further use
There are also requirements for customers to be informed when they have reached 80 per cent and 100 per cent of their data allowance.
And operators must take “reasonable steps” to avoid customers being charged for accidental roaming in Northern Ireland, which would happen if a phone locked onto the mobile signal coming from the Irish Republic.
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