Vodafone has apologised after an error meant customers using the mobile network abroad were hit with roaming bills of up to £5,000.
Customers took to Twitter, saying they were unable to use their phones and could not reach the operator.
Many received alerts from Vodafone that their data had run out, despite the fact that many still had data remaining in their monthly allowances.
Vodafone said the issue was caused by a technical error.
Since 2017, under EU regulations UK consumers are able to use the minutes, texts and data included on their mobile phone tariffs when travelling in the EU at no extra charge.
Vodafone customers posted on Twitter that their bills had risen by between hundreds and thousands of pounds within just 12 hours.
Another user tweeted: "So upset!! Have spent nearly 2 hours getting through to your support line whilst@on holiday in Italy, ruined my Sunday night.
"£910 bill and thought I'd done something wrong, burst into tears! Really not happy Vodafone."
Vodafone said: "We are very sorry that some customers could not use their phones yesterday, when roaming abroad. This was due to a technical error, which we have now fixed.
"Some customers are receiving billing messages in error; we are working through these as an urgent priority and are removing any errors from customer accounts. Customers will not be charged and do not need to worry about contacting us as we are proactively checking accounts."
In August, a similar issue affected Three Ireland. The mobile operator had to apologise after a system upgrade error added bogus roaming fees to customers' bills and suspended their services.
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