The UK terror threat has been raised to the highest level following the attack on a tube train in Parsons Green today. (15 September.)

Speaking from 10 Downing street British PM Theresa May said the threat had been raised from 'severe' to 'critical', meaning a further attack is expected imminently.

"I said earlier today that the national threat level was being kept under review. The Joint Terrorism Analysis Centre, that is the independent organisation that is responsible for setting the threat level on the basis of available intelligence, has now decided to raise the national threat level from severe to critical.

"This means that their assessment is that further attack may be imminent, "she explained.

The PM said the military will support the police and will be deployed to key sites in the capital.

"Police have asked for authorisation from the Secretary of State for defence to enact the first phase of Operation Temperer, a well-established plan to provide military support to the police," she said.

Theresa May
Prime Minister Theresa May has urged the public to go about their business as usual but remain vigilantReuters

Military personnel will replace police at certain protected sites, inaccessible to the public. The presence of the armed forces personnel on British streets and the underground will provide "protection and reassurance to the public", the PM said.

The PM urged the public 'to go about their business as usual, but to be vigilant and cooperate with police.

Police and emergency services were called to Parsons Green station at 8.20am following reports of a fire on a District line underground train.

Eye-witnesses reported hearing an explosion and seeing a large fireball in a carriage, which sparked panic on the packed train.

Parsons Green District Line
A bucket with a burning item inside it and wires coming out of it is seen on the floor of an District Line train carriage at Parsons Green stationSylvain Pennec/Reuters

Photographs show a Lidl shopping bag with what appeared to be a bucket with wires protruding from it. Explosive experts said the home-made bomb, which only partially exploded, may have been detonated using a timer.

29 people have been treated in hospital, most suffering with burns, while others were injured in the stampede as passengers tried to flee the train carriage.

As a manhunt continues for the attacker who remains at large, Metropolitan Police Assistant Commissioner Mark Rowley has said police are "chasing down suspects".

While there have been no arrests so far, he said that the police are making "excellent progress" as "hundreds of police officers are trawling through CCTV".

Following a statement on the Amaq news agency in which Islamic State claimed responsibility for the Parsons Green attack, Rowley warned that IS frequently claim to have carried out attacks irrespective of whether they were involved or not.

He added that the raised terror threat will result in an extra 1000 armed police on the streets of London.

He urged members of the public to "act on their instinct" if they saw anything concerning, and "not to hesitate" to call the police to report anything suspicious.

Responding to the PM's announcement that the national threat level has now been raised to 'critical' London Mayor Sadiq Khan issued a statement urging the public to " remain vigilant,!

He said: " I utterly condemn the hideous individuals who target innocent people and attempt to use terror to harm us and destroy our way of life. Please remain vigilant. If anybody knows anything at all about who might be responsible for this attack, they should contact the police immediately. The Anti-Terrorist Hotline is 0800 789 321."

He attempted to reassure the public that the city of London and its people will be protected.

"Everything possible is being done to protect our city and keep us all safe, both Londoners and visitors to the capital. Rest assured the full resources of our police and security services are being deployed to track down those responsible. They will be caught and brought to justice," he said.

Calling on the city to unite in the face of terror he added: "London stands together today as always. We will never let those who seek to divide us win – we will never be intimidated or defeated by terrorism."

It is the fourth time that the UK terror threat level has been placed at "critical" in the past 11 years.

The terror threat was last at "critical", rather than severe, after the Manchester Arena terror attack in May which left 22 concert-goers dead.