The UK’s biggest mobile network will bill customers £3.50 for every gigabyte of data used above a 25GB limit.

It announced the new charges in an email to affected customers, saying it was a change to the network’s “fair usage policy”.

British holidaymakers have benefited from free roaming costs in the EU since “Roam like at home” rules were introduced in 2017. The EU rule meant that UK mobile customers could use their mobile call, text and data allowance in Europe without incurring extra costs.

But Brexit means the rule no longer stands. O2 is the first major network to reintroduce roaming charges since the UK left the EU. In 2019 it said it didn’t have any plans to re-introduce roaming charges post-Brexit.

An email to O2 customers with a monthly data allowance of 25GB or more said: “As your monthly UK data allowance is over 25GB, you can still use your data in our Europe Zone. 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.”

Ernest Doku, mobiles expert at Uswitch.com, said: “Holidaymakers thought they had seen the back of European roaming charges for good, so the return of data roaming caps is hugely disappointing.

“The fear is that moves to introduce charges will spark a domino effect that sees every supplier follow suit and data charges ramp up. If you’re a customer of one of these providers and are unhappy about these charges, remember you can leave without paying a fee if you’re out of contract.

“When it comes to travelling, don’t leave it till the last minute to check the roaming charges for your destination, and always use hotel and cafe Wi-Fi when on holiday where possible.”