Britain’s biggest mobile network has billed customers thousands of pounds and blocked their phones due to a system error.

A glitch at EE, part of BT Group, has meant customers racked up vast debts. The issue, which started last week, has affected users travelling in America, Canada, Mexico, Australia and New Zealand.

One customer said they had been incorrectly charged £25,000 while another had unwittingly racked up more than £4,500 after four weeks in the US. Many said they feared the erroneous bills would be automatically taken from their bank accounts. 

Users have received text messages telling them to clear their debts or network access would be cut off. Blocked customers still cannot not use their phones, despite contacting EE to rectify the issue. Some have had to wait more than a week.

Bills built up despite customers buying “roam abroad” passes. The £10.44-a-month add on is supposed to allow customers to use their phones as normal while overseas but the purchase was not recogonised by the network.

Elizabeth Williams, 38, a Londoner who works in the film industry, was on a four-day business trip to Los Angeles when she received a text message saying she had built up £500 in charges.

“It said I needed to pay or my phone would be shut down. It was a massive shock at six o’clock in the morning while I was relying on my phone for calls, meetings, emails and directions,” she said.

“I rang them up straight away. They told me the issue was fixed but the charges are still showing up,” she added.

EE, which claims to be Britain’s best, fastest and most reliable network, has apologised to those affected. It promised it would remove the charges before bills were produced, adding it was “aware of the incident”.

A spokesman said the “problem” that had allowed the bills to pile up in error had been quickly fixed and said customers could now use their roaming passes abroad as normal. “We are contacting affected customers to reassure them that they won’t have to pay for these incorrect charges,” he added.