Plusnet investigates top-up data billing faults
BT's Plusnet home broadband service has apologised after some of its customers incorrectly received emails telling them they were being billed extra for going over their traffic allowance.
The problem began on Tuesday, when subscribers' accounts started adding gigabytes of unused data.
Plusnet said that its engineers were "investigating the root cause".
It added that the issue had only affected "a small number" of its users, but did not say how many.
One customer contacted the BBC to report the problem.
"At 22 minutes past midnight this morning I received an email, and it said you are approaching your usage allowance, and once you've used your 40GB we'll charge you an extra £5 for every 5GB," Steve Rogers said.
"And then at 8.42am I got an email saying you've exceeded your usage, we've added some more.
"And then it happened again at 9.51am.
"I thought I haven't been doing anything out of the ordinary... I wonder if my wife has been downloading loads of stuff."
A check of Mr Rogers' account, using Plusnet's View My Usage tool, revealed that 28.2GB of data had been added to his tally yesterday and a further 14GB on Wednesday morning - far in excess of his actual usage.
Plusnet has put an answerphone message on its hotline and a note on its support page acknowledging the problem.
A spokeswoman also issued a statement to the BBC.
"Yesterday, a small number of Plusnet customers on 'limited' broadband packages received email notifications regarding their broadband usage," it said.
"The emails incorrectly stated that customers had gone over their usage limits. The issue has now been resolved.
"We are confident that no customers have been overcharged. However, if any customers have any concerns, please visit the Member Centre on our website or feel free to get in touch with us."
Plusnet's service page notes that maintenance work was carried out on its back-end systems on Sunday evening, but the firm said that this was not linked to the fault.