BT has apologised after "several hundred thousand" UK customers had their broadband services cut off.
Down detector, a website that monitors internet failures, reported thousands of cases, including in Glasgow, London, Birmingham and Sheffield.
At about 23:00 GMT BT said it was confident services had been "fully restored" but that some customers might need to reboot equipment.
The company said a faulty router was to blame for the problem.
BT phone services were unaffected by the outage, which began about 14:30 GMT.
A spokeswoman added: "Most customers will be able to access their broadband without taking any action but some may need to reboot their equipment."
Hundreds of people took to social media to express their frustration using the hashtag #BTdown, which began trending on Twitter.
BT acknowledged that "large numbers of customers" were "experiencing temporary issues with their broadband services".
It did not initially give details of what may have caused the issue but said: "There is no evidence at this stage to suggest that we were subject to a malicious attack."
Its website had also crashed for a time and some users reported that its customer service line was down.