The Prime Minister raised Britain’s terror threat level to the highest possible stage, meaning another attack could be expected imminently.
It came after the terror group ISIS claimed responsibility for the bombing on a packed rush hour train at Parsons Green in west London during the morning rush hour which left 29 people injured. The Prime Minister said military personnel would replace police officers “on guard duties at certain protected sites which are not accessible to the public”.
Mrs May said: “The Joint Terrorism Analysis Centre (JTAC) has now decided to raise the national threat level from severe to critical.
“This means that their assessment is that a future attack may be imminent.
“Following JTAC’s decision police have asked for authorisation from the Secretary of State for Defence to enact part of the first phase of operation Temperer.
“This is a well established plan to provide military support to the police and for this period military personnel will replace police officers on guard duties at certain protected sites which are not accessible to the public.
“The public will see more armed police on the transport network and on our streets providing extra protection and this is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses.
“The public should go about their business in the normal way and as usual be vigilant and cooperate with the police.
“I’ve said earlier that terrorism is a great challenge of our times but by standing together we will defeat it.”
It was reported that the bombing was the work of a “detachment” of ISIS, rather than a “soldier” acting alone.
Scotland Yard has launched a huge counter-terrorism investigation alongside MI5, while the device is being forensically examined.
Pictures posted on social media appeared to show wires protruding from a flaming bucket inside a plastic carrier bag on the floor of a carriage.
The suspected device, apparently a builder’s bucket filled with a white substance that had been placed in a Lidl bag, was seen in flames afterwards, with wires hanging out of the bag. One theory is that the device failed to fully detonate.
Detectives are scouring CCTV and examining witness statements for clues in their hunt for one or more suspects.
Mark Rowley, Assistant Commissioner for Specialist Operations of the Metropolitan Police Service said: “We are making excellent progress at the moment as we pursue our lines of enquiry to identify, to locate and to arrest those responsible.
“This is a very complex investigation which is continuing at speed with the full weight of the London counter terrorism’s policing resources assisted by colleagues across the country and by our intelligence agency partners at M15.
“We have hundreds of police officers trawling through CCTV footage, detectives have spoken to tens of witnesses and we’ve taken a number of calls to the anti-terrorism hotline from members of the public.”
Members of the public have sent in so far 77 images and videos they’ve taken at the scene which are being assessed, he said.
“Meanwhile the improvised explosive device on the train, the remnants of it have now been made safe and they’ve been taken away for specialist examination by forensics scientists.
“Whilst we chase down the suspects we are strengthening our policing resources on the streets of London and across the country.
“The public should expect to see extra policing resources armed and unarmed on the streets, additional stop and search using counter terrorism powers and potentially additional road checks.