The issue affecting users in the south east of England and in Cardiff, the company said on Twitter.
It assured anyone affected that “engineers are on site and working to restore service” as soon as possible, but said it was unable to give an estimated time of how long the issue would take to fix.
The problem was the result of “an issue at exchanges”, Sky said, though it gave few details on what exactly that problem might be.
The issues appeared to be primarily affecting users in and around London, in Brighton, and in Cardiff and other parts of south Wales, according to tracking website Down Detector.
Sky also advised those affected to keep an eye on its customer services page, where users can check their internet connection and will be given updates on when the problem is fixed. That page does however, require an internet connection.
As with many other large internet providers, Sky is part of Ofcom’s voluntary scheme to provide automatic compensation to anyone hit by internet issues that last for a long time. Sky will pay customers £8.06 per day during an outage.
That scheme does however require the problems to last for two full working days, and the latest issues only appear to have started late on Wednesday. As such, compensation will not be due unless the fault is not fixed by the very end of Friday.
The problems appear to have begun before 6.30pm on Wednesday evening, at which point Sky warned customers that they “might not be able to get online or make/receive phone calls due to a problem in [their] local area”.
“Our technical teams will continue to work overnight to fix the problem and we will provide an update on progress in the morning,” it said at that time. “We’re sorry for the continued inconvenience.”
On Thursday morning it posted another message indicating that it had “identified the problem” and that its “engineers continue working hard to restore service as soon as possible”. It apologised again for the problems.