Thousands of members of the armed forces could be sent out onto the streets to protect Britain after the terror threat level was raised to critical in the wake of the Manchester suicide bombing.
Theresa May has activated Operation Tempererer, which allows for armed members of the services to temporarily take on a key role in guarding major sites and big events.
Here's everything you need to know about Operation Temperer and what it involves.
What is Operation Temperer?
It is a plan devised by the government to help protect civilians at times of high terror threat by bringing in members of the army to help protect key sites at risk of attack such as transport hubs, stadiums and key landmarks.
The plan was first revealed in 2015. Home Secretary Amber Rudd said it would allow for up to 3,800 members of the armed forces to be deployed throughout the UK to support police.
Ms Rudd stressed that the move was an "absolutely temporary" response to the immediate threat.
All troops on the streets will be working under the command of police officers.
Where will the armed troops be deployed?
The exact details of deployments have not been revealed - and they are unlikely to be made public for reasons of security.
However, Britons are likely to see armed troops at major transport hubs such as airports and large train stations.
Mrs May has also said they may well also be deployed at major sports or entertainments events like the FA Cup final at Wembley on Saturday or the Champions League final in Cardiff on June 3.
Met Police Assistant Commissioner Mark Rowley said the first phase of the operation would allow soldiers to replace police officers at fixed locations but after that they could "augment our patrols" at major events.
Why has Operation Temperer been activated now?
The decision was taken by Mrs May after the official terror threat level was raised to "critical" - the highest level of risk. It warns that a further terrorist attack on UK soil is feared to be "imminent".
She said that it will "allow the police to significantly increase the number of armed officers on patrol in key locations" in a speech announcing that the operation was now in effect.
Mr Rowley, the country's senior counter-terrorism officer said acknowledged that the use of troops was "unusual" but it would allow police to "stretch our armed capability".
How significant is the raise in the terror threat level?
It's extremely unusual and means an attack is expected imminently.
It is only the third time that the risk has been placed at the highest level since the scale was made public in 2006.
The others are:
August 2006 after a plan to bomb up to 10 transatlantic flights was uncovered
June 2007 following a plot to bomb a club in central London
On both occasions the assessment was lowered after a few days.
Previously the level was assessed to be 'severe' - the second-highest of its five possible settings.
The other levels are: substantial (an attack is a strong possibility); moderate (an attack is possible but not likely), and low (an attack is unlikely).
Who decides the terror threat level?
An organisation called the Joint Terrorism Analysis Centre, which based at MI5's London headquarters and brings together representatives from 16 government departments and agencies.
Together they agree the threat level based on a number of factors including available intelligence, terrorist capability, terrorist intentions and timescale.