The level has only been Critical twice before
Britain has raised its threat level to the highest point, following a suicide bomb attack on the Manchester Arena on Monday night.
The attack at an Ariana Grande concert left 22 people dead and another 64 injured.
Prime Minister Theresa May announced Tuesday that her country's terror threat level has been raised from severe to critical.
But what does this mean?
Intelligence agency MI5 says the levels are designed to give a broad indication of the likelihood of a terrorist attack.
They range from Low - meaning an attack is unlikely - to Critical, meaning an attack is expected imminently.
It will see the British military providing armed guards for key locations - including Buckingham Palace, Downing Street, the Palace of Westminster and embassies in London.
The Metropolitan Police say the public will see more armed officers "on proactive operations and targeted patrols".
It says the locations of their deployments, types of tactics and numbers on duty will continually change.
The military will be working under the command structure of the Metropolitan Police.
The threat level for the UK is set by the Joint Terrorism Analysis Centre (JTAC).
JTAC was established in June 2003 and is based in MI5 headquarters in London.
It is a self-standing organisation made up of representatives from 16 UK government departments and agencies.
MI5 is also responsible for setting terror threat levels from Irish and other domestic terrorism both in Northern Ireland and in Great Britain.
The current threat level for Northern Ireland-related terrorism in Britain is Substantial.
MI5 says a judgement on the threat level has several factors.
These include available intelligence, terrorist capability, terrorist intentions and timescale.
MI5 says: "The threat level expresses the likelihood of an attack in the near term.
"We know from past incidents that some attacks take years to plan, while others are put together more quickly.
"In the absence of specific intelligence, a judgement will need to be made about how close an attack might be to fruition.
"Threat levels do not have any set expiry date, but are regularly subject to review in order to ensure that they remain current."
Since 2006, information about Britain's terror threat level has been available on the MI5 and UK Home Office websites.
While threat levels for Northern Ireland-related terrorism were made available from September 2010.
The UK's terror threat level has only been set to Critical twice before: August 2006 and June 2007.
It was raised in 2006 after a plot to kill thousands of people by detonating explosions on up to 10 transatlantic flights from UK airports was disrupted.
The level was raised a second time in 2007 after a blazing car loaded with propane canisters was driven into a crowded Glasgow Airport.
The levels were brought in a year after the July 7th bombings, in which 52 people were killed and more than 700 injured in London.