Members of the armed forces will be based at key sites including Buckingham Palace, Downing Street and the Palace of Westminster after the UK Government raised the terror threat to critical.
The UK terror threat level wasraised to the highest level, meaning further attacks may be imminent, following Monday nights attack in Manchester.
Prime Minister Theresa May said the decision was taken after investigators weren’t able to rule out that the suspect Manchester bombing suspect Salman Abedi did not act alone.
It is the first time the threat has been raised to the highest level since 2006.
The change means military personnel will be authorised to take over certain security duties formerly taken care of by the police and will be used to boost security at large public gatherings and at busy locations such as airports and train stations.
It also means more resources will be given to the police to investigate and prevent terror threats.
Monday night’s attack at the Manchester Arena left 22 dead, including children and teenagers, and 59 injured.
Sixty nine people are still being treated in hospitals across Greater Manchester and 20 are in a critical condition.
This morning it was announced a 15-year-old girl, Olivia Campbell, is among the dead, as were a Polish couple who it is believed were waiting to meet their children after the concert.
Home Secretary Amber Rudd said the Manchester attack was ‘more sophisticated’ than other incidents.
When asked on Radio 4’s Today programme how long the critical level would remain, she said: "The last time the threat level was raised to critical lasted five days and before that, four days, so we will see."
London Mayor Sidiq Khan said: "We are doing all we can to keep Londoners and visitors safe."
Manchester Mayor Andy Burnham said key events, such as the Great City Games, due to take place this weekend, will still go ahead.
Among the missing is a 17-year-old part-time travel agent apprentice, Chloe Rutherford, an employee at Westoe Travel in South Shields.
