Channel says it is working to restore pictures ‘as quickly as possible’
On-demand services such as More4 are also affected, with many viewers unable to watch programmes.
It is unclear what caused the outage or how long it will last.
Social media users reported the problem was caused by a fire alarm at a transmission facility in White City, the former BBC headquarters which is now used by independent studios and production companies.
“Channel 4 and More 4 are currently off air due to a technical problem,” the channel tweeted. “We are working to restore them as quickly as possible.”