In response to the explosion on a Tube train at Parsons Green station, Britain's military will be deployed to support armed police under Operation Temperer.
What is Operation Temperer?
It is an established security plan by the Government which aims to help protect civilians at times of high terror threat.
The official national threat level has been raised from 'severe' to 'critical', meaning that an attack is expected imminently.
On request from the police, and with authorisation from the Defence Secretary, the first phase of Operation Temperer is now being partially enacted.
The threat level has been increased to its highest level only three times before - first in 2006 after a liquid bomb plot was discovered, then in 2007 after a failed attack to gas cylinders outside a Central London bar and in May following the Manchester attack.
On those occasions, it was reduced after a few days.
What does it mean?
During this temporary deployment, soldiers working under police command will replace armed police officers to guard unnamed infrastructure sites across the country.
This will allow an increase in the number of armed officers on patrol in key locations.
As this is just the first phase of Operation Temperer, police chiefs will decide where to deploy the military and soldiers will not take over.
Troops may be covertly moved around the country.
The plan has been enacted after warnings that another terrorist attack could be imminent.
Operation Temperer was first revealed in 2015 and it is meant to be a temporary measure.