The issue meant that connectivity repeatedly dropped out over Monday evening and into the early hours of Tuesday morning, leaving customers without their usual service.
Now it says the problem has been fixed – and noted that it was not the result of increased usage owing to the coronavirus lockdown.
"An intermittent broadband issue that started yesterday evening was fixed in the early hours of this morning," a spokesperson said.
"This was not caused by increased usage or a lack of capacity. We know how frustrating this was for customers and we sincerely apologise for any inconvenience caused."
The issue began in the early evening on Monday, as users found that their internet connections dropped out and their computers were unable to get online. While some found that the network appeared to restore itself, it then dropped back out again, with the issue happening repeatedly.
The problems hit users across the country, most of whom are stuck working or studying at home or attempting to keep themselves entertained through the lockdown.
Most internet service providers have seemingly coped with any extra demand or changes in usage as a result of coronavirus measures. Outages have been rare, and companies have said that traffic has surged less than might have been expected.
Some services, such as Netflix and PlayStation, have limited the speed or quality of their streams and downloads in an attempt to limit the impact on internet infrastructure.
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