British security services' decision to raise the threat level from "severe" to "critical" means that the UK's army can deploy up to 5,000 troops across "sensitive" security locations, such as the Houses of Parliament, in the wake of the Manchester terror attack.
The decision to raise the terror threat to its highest level came from Joint Terrorism Analysis Centre (JTAC) and means "an attack is expected imminently", Prime Minister Theresa May announced on Tuesday evening (23 May).
The threat level has only hit "critical" twice before since its introduction in 2006 – the first time was in reaction to the foiled transatlantic aircraft plot, when terrorists planned to use liquid explosive devices.
The second was in response to two cars laden with explosives being discovered outside a nightclub in London's Haymarket in 2007.
The Defence Secretary Michael Fallon has since authorised the roll-out of Operation Temperer, formulated in 2015, for the first time as the authorities continue to investigate whether suicide bomber Salman Abedi, 22, was part of a terrorist network.
"This means that armed police officers responsible for duties such as guarding key sites will be replaced by members of the armed forces, which will allow the police to significantly increase the number of armed officers on patrol in key locations," May said.
"You might also see military personnel deployed at certain events, such as concerts and sports matches, helping the police to keep the public safe. In all circumstances, members of the armed forces who are deployed in this way will be under the command of police officers."
The decision comes just days before major events across the UK on the weekend, including the FA Cup final at Wembley Stadium and the Premiership Rugby final at Twickenham.
"The police will work with the organisers and hosts of these events, to come to a judgement about how they can go ahead while making sure the people who attend them are safe and secure," the prime minister said.
There is no expiry date for the "critical" threat level, but in 2007 the authorities lowered it back down to "severe" after four days.