The telecoms regulator found the company had broken fundamental billing rules.

Users who called the company’s ‘150’ customer services number while roaming within the EU were incorrectly charged as if they had called the US.

This meant customers were charged £1.20 per minute, instead of 19p per minute. As a result, at least 32,145 customers were overcharged around £245,700 in total.

Ofcom said EE did not reimburse the majority of affected customers until the regulator intervened.

Lindsey Fussell, Ofcom’s consumer group director, said: “EE didn’t take enough care to ensure that its customers were billed accurately. This ended up costing customers thousands of pounds, which is completely unacceptable.

“We monitor how phone companies bill their customers, and will not tolerate careless mistakes. Any company that breaks Ofcom’s rules should expect similar consequences.”

The fine will go the Treasury.

Most customers have been refunded but EE was unable to identify at least 6,905 customers, who were more than £60,000 out of pocket in total.

EE has made a donation of just under £62,000 to charity in lieu of the payments owed to these customers.

An EE spokesperson said: “We accept these findings and apologise unreservedly to those customers affected by these technical billing issues between 2014 & 2015. We have put measures in place to prevent this from happening again, and have contacted the majority of customers to apologise and provide a full refund.”