The BBC's iPlayer went down on Thursday, meaning that nobody can access one of the most popular video services in the world.
The service was hit by an outage that appeared to affect every digital or online BBC service. Its radio and TV broadcasts were unaffected.
The corporation's press office said that it was "aware of a technical issue affecting the BBC website and are working to fix this now," in a tweet. "We'll update you as soon as we can," it said.
Another tweet from the BBC's breaking news team said that it was "working as quickly as possible to resolve" the problems.
The Down Detector website showed that reports of problems began soon after 6am UK time, and carried on for much of the morning.
Many websites count iPlayer as part of the corporation's main website. But that site is the 89th biggest in the world, and the seventh-ranked site in the United Kingdom.
According to BBC News, sources inside the corporation said a distributed denial of service (DDoS) attack, which aims to knock a website offline by flooding it with web traffic until it can no longer cope and crashes, was the cause of the problem.
No one has claimed responsibility for the attack.
The corporation itself has declined to offer any further comment on the incident, other than confirming there was a technical issue that had now been solved, and that the various sites were now "operating normally".
The BBC's range of sites have been targeted before - in July 2014 the iPlayer service was offline for an entire weekend after a fault developed in a database used by the on-demand TV service.