Critical threat level: who made the decision and what does it mean?

The first visible signs of the deployment of 984 soldiers on the streets of Britain will be on Wednesday afternoon outside Downing Street, the Houses of Parliament and Buckingham Palace as they “backfill” guarding duties at key sites currently carried out by armed police officers.

Both the prime minister and home secretary made clear in their announcement of the move that they are extremely wary of the political impact of the sight of armed soldiers patrolling the streets of Britain and want to use them for as short a time as possible.

The threat level has only twice before been raised to critical since the system of official threat levels was introduced on 1 August 2006. It was raised for three days from 10 August 2006 at the time of the transatlantic airliner plot, and on 30 June 2007 when the security services uncovered the plot to bomb a nightclub on London’s Haymarket.

This was a plot that culminated in the attack on Glasgow airport. The threat level was reduced to severe three days later on 13 August 2007 after suspects were arrested as far afield as Australia. However, already there are warnings to the police that this time the critical threat may last up to a week.

The use of troops “in aid to the civil power” on the streets of Britain is a major move by the security chiefs and directly flows from the decision to raise the threat level from severe – meaning an attack is likely – to critical, meaning an attack is expected imminently.

As Theresa May explained when she made the decision public: “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.

“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.” Sports events are expected to include this weekend’s FA Cup final.

The announcement of the move to a critical threat level on Tuesday night was unusual. In recent years changes in the threat level have been announced by the home secretary alone on a recommendation from the “independent” Joint Terrorism Analysis Centre (JTAC).

The decision was also unusual in that MI5’s definition of a critical threat is that an attack is expected imminently.

May did not say this in her late-night statement. Instead she said only that it was “a possibility we cannot ignore that there is a wider group of individuals linked to this attack” and that “a further attack may be imminent”. This is different and does not mean that they know an active cell, including a potential bomber, is at large but that they fear it may be the case and need to be cautious.

For both May and Amber Rudd one major reason for their anxiety will be that the sight of armed troops on the streets will provoke accusations of moves towards a French-style “militarised police state” and inevitable speculation that in the middle of a general election campaign the decision comes with political overtones as well as from operational necessity.

The last time the use of troops under Operation Temperer was considered was immediately after the terrorist attacks in Paris. But in the face of defence reservations they were only used covertly as backup for the visible armed police presence at railway stations and airports, and so this is the first time the plan has been fully activated.

Both May and Rudd will be also be mindful of the reaction to a 2003 decision by Tony Blair to deploy 400 soldiers and three Scimitar armoured cars at Heathrow airport in response to an alleged plot for a missile attack on aircraft. Blair was sharply criticised for his “send in the tanks”-style response to a terrorist threat.

Defence chiefs are also anxious about the use of their troops on the streets. They look at France where an initial deployment in the immediate aftermath of the Brussels and Paris attacks has proved far from temporary. Indeed, their numbers have since been increased and armed soldiers patrolling the streets are now a common sight even in towns in rural France. The military has also voiced concerns about overstretch and the morale of the troops being used.

Operation Temperer means that up to 3,800 armed troops can be deployed at sensitive sites and events, transport hubs and other crowded public places in order to release the armed police for other policing duties. They can only be used “to augment armed police officers engaged in protective security duties” and can only be authorised by a Cobra emergency meeting.

The prime minister went out of her way to stress that the armed troops would be under the direct command of the police who would also determine where and how they are used. The home secretary also stressed it was a temporary measure.

The decision to increase the threat level was taken by JTAC, which analyses and assesses all intelligence relating to international terrorism in Britain and overseas. It brings together counter-terrorism experts from the police, security services and Whitehall departments. They include Charles Farr, chairman of the joint intelligence committee, and Mark Sedwill, national security adviser, who both worked very closely with May at the Home Office.

As May indicated, it met repeatedly on Tuesday to assess the level of threat. In the morning it decided that the threat level should remain at severe but as the police investigation continued into the associates of the Manchester suicide bomber – Salman Abedi – they took the decision to raise it to the highest level.

As the prime minister explained, the “possibility of a wider network” being behind Abedi’s attack justified their decision. The clear implication is that he came from a terrorist jihadi cell some elements of which may well still be active and a real danger.