Emergency 999 calls across the country have not been connecting amid technical issues with the phone lines.
People have been urged to call 101 in an emergency for police and fire, or 111 in a medical emergency, after problems were reported on Sunday morning.
Several police forces and fire and ambulance services said they were facing issues and said BT had reported a “system failure”.
A spokesperson for the Metropolitan Police said: “Due to a technical fault that is impacting a number of police forces, many 999 calls are not connecting.
“Until further notice, anyone in London who requires the police in an emergency is asked to call 101.
“Please only call in an emergency and please wait until later to make any 101 non-emergency calls.”
The spokesperson confirmed that BT runs the 999 system.
Other forces reporting issues included Greater Manchester Police, Hampshire Police, Essex Police, Bedfordshire Police, West Yorkshire Police, West Midlands Police, Norfolk Police, South Wales Police and Police Scotland, among others.
Fire services, including Bedfordshire and Leicestershire, also warned of the fault, as did ambulance services such as the East of England Ambulance Service.
An NHS spokesperson said: “There is an issue affecting the national 999 call system.
“BT is working to resolve this issue as soon as possible but, in the meantime, if you are unable to contact 999 in a medical emergency, please call 111.”
It is not known how many emergency services have been affected.
Some services reported the problems had been resolved at around 9.30am, but BT has yet to comment.
