In response to the terror attack in Manchester, 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 decision was taken by the Prime Minister and authorised by the Defence Secretary after the official national threat level was raised from 'severe' to 'critical', meaning that an attack is expected imminently.
The threat level has been increased to its highest level only twice before - first in 2006 after a liquid bomb plot was discovered, and then in 2007 after a failed attack to gas cylinders outside a Central London bar.
On those occasions, it was reduced after three and four days, respectively.
Theresa May has suggested that this time it could last several weeks.
What does it mean?
During this temporary deployment, soldiers will be working under police command and replace armed police officers to guard major sites such as rail stations, Parliament, embassies and Buckingham Palace.
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.
Met Police Assistant Commissioner Mark Rowley said that police are working "really closely" with event organisers to review security in the upcoming weeks.
Theresa May has said that troops will also be present at major sporting or entertainment events, such as the FA Cup final at Wembley or the Champions League final in Cardiff.
Other troops might be covertly moved around the country.
Home Secretary Amber Rudd said 984 troops have been deployed in the first instance and up to 3,800 are available.
The plan has been enacted after warnings that another terrorist attack could be imminent because a "wider group of individuals" could have been involved in the Manchester attack.
Speaking on Tuesday evening in a live televised statement, Theresa May said she does not want the public "to feel unduly alarmed":
"The operational response I have just outlined is a proportionate and sensible response (...) I ask everybody to be vigilant, and to cooperate with and support the police as they go about their important work."
Operation Temperer was first revealed in 2015 and it is meant to be a temporary measure.
When was it last used?
It was reported that troops were deployed under the operation after the Brussels terrorist attacks in 2016. This was never confirmed by the Ministry of Defence.