Manchester attack latest: Armed soldiers arrive on Britain's streets amid fears of second terror plot

British soldiers enter a Ministry of Defence building near New Scotland Yard police headquarters.

Police forces outside London have also asked for troops.

Amber Rudd, the Home Secretary, said: “We have now gone to critical level in terms of the threat. Op Temperer has been invoked and that means that there will be additional military personnel coming to back fill the armed police officers so that they can support other areas.

“Today we have 984 members of the military coming forward as requested by police.”  

Scotland Yard said the arrest yards from Buckingham Palace was 'not believed to be terror-related' 

It comes as a man with a knife was arrested near Buckingham Palace on Wednesday. The man was detained by police on The Mall just moments before the Queen passed by in a car on her way to an engagement with the Duke of Edinburgh at St Paul's Cathedral.

A picture showed the man surrounded by officers and handcuffed as he sat on the ground, while a police officer nearby held what was believed to be a large blade that had been seized.

The Metropolitan Police said the incident, at around 10.40am, is not believed to be terror-related. However, it comes amid heightened security and as armed soldiers prepare to guard "key locations" across London.

A police officer holds what is believed to be a large knife that is thought to have been seized from the man outside Buckingham Palace CREDIT: CARL COURT/GETTY

Earlier, the Ministry of Defence announced that the Changing the Guard ceremony at Buckingham Palace had been cancelled today to redeploy police officers.

The authorities are seeking to establish if Abedi, the Manchester suicide bomber who left 22 people dead, was part of a wider network.

Manchester Arena bomber Salman Abedi

Greater Manchester Police revealed on Wednesday that three men have been arrested in the south of the city in connection with the concert bomb attack. It follows the arrest yesterday of a 23-year-old man, who remains in custody.

Amber Rudd, the Home Secretary, said on Wednesday that it is "likely" Abedi was not acting alone as she confirmed up to 3,800 troops can be deployed across the UK.

Prime Minister Theresa May has raised the threat level to the highest possible rating, meaning another atrocity is expected. She said a "wider group of individuals" could have been involved in the Manchester Arena blast rather than just Abedi.

 1,000 British soldiers are being immediately deployed on the streets to support police in the wake of the Manchester suicide bombing, as the UK remains on critical terror alert amid fears Salman Abedi did not act alone.

Armed troops will take on duties for guarding embassies and high profile buildings, freeing up armed police officers to join counter terrorism duties.

Soldiers are due to take up position at Buckingham Palace, Parliament and Downing Street later on Wednesday as part of the deployment named Operation Temperer, which is being enacted after security experts warned the Government that another terrorist attack could be imminent.