London - Britain raised its national threat level to maximum on Friday after a bomb detonated on a London Underground train, and Prime Minister Theresa May said troops would be deployed to key locations.
The terror threat has been increased from "severe" to "critical", meaning another attack is expected imminently, after the explosion on a packed morning rush-hour train left 29 people injured.
In a pre-recorded television statement, May said military personnel would replace police officers "on guard duties at certain protected sites which are not accessible to the public".
She said: "The public will see more armed police on the transport network and on our streets, providing extra protection.
"This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."
The Islamic State group has claimed responsibility for the explosion, Britain's fifth terror attack in six months.
The terror threat level was raised to "critical" in May after the Manchester bombing, which was also claimed by the ISIS group.