Armed troops were sent into the Palace of Westminster today after Theresa May raised the terror threat level in the UK to “critical”.

Parliament was also closed to the public following police advice in the wake of the Manchester bomb attack – adding to the feeling of apprehension.

Around 30 soldiers patrolled both the streets outside Parliament and entered the landmark building, after the Prime Minister announced that the Army would be deployed to protect key sites. Troops were also seen in Downing Street.

The extraordinary scenes followed security service advice to raise the threat level to its highest for the first time in 10 years, meaning another attack is feared to be imminent.

Under Operation Temperer, up to 5,000 soldiers are being deployed to transport hubs and other crowded public places, in order to release the armed police for other duties.

It is the first time troops have been used in this way since Tony Blair was criticised for sending 400 soldiers and a number of armed vehicles to Heathrow in 2003.

Ms May has sought to calm fears by stressing that the armed troops will be under the direct command of the police.

The recommendation to move to “critical” was made by the Joint Terrorism Analysis Centre, which brings together counter-terrorist expertise and Government departments.

It came as police in Manchester worked to establish whether the bomber Salman Abedi, 22, was working alone when he killed 22 people and injured 59 others in Monday night’s suicide explosion.

Soldiers arrive at Parliament as the UK terror threat was raised to critical (Kate Proctor/Evening Standard)

Last night, the Prime Minister said: “The work undertaken throughout the day has revealed that it is a possibility that we cannot ignore, that there is a wider group of individuals linked to this attack.”

She added: “As a result of the decision, the police have asked for authorisation from the Secretary of State for Defence to deploy a number of armed military personnel in support of their armed officers.

“You might also see military personnel deployed at certain events such as concerts and sports matches, helping the police to keep the public safe.”

Meanwhile, a statement on Parliament’s website said all tours and events were being cancelled with immediate effect.

“In light of the tragic events in Manchester and the Prime Minister's announcement raising the national security threat level to critical, the police have advised that the Palace of Westminster should be closed to non-passholders today, Wednesday 24 May,” read the statement.

“As a consequence all tours, events and banqueting will be cancelled with immediate effect. This arrangement will remain in place until the advice changes.”

There are currently no sitting MPs following the Prime Minister’s decision to stage a snap general election. Parliament is not due to sit again until June 19.

Election campaigning remains suspended, although pressure grows for it to resume no later than Friday – and possibly as early as tomorrow.