The prime minister raised the threat level from severe to critical for the first time since July 2007, meaning “not only that an attack remains highly likely but a further attack may be imminent”.
May said she did not want to “unduly alarm” people but military personnel would be present at public events and key sites under police command.
She said it could not be ruled out that the attacker, named as Salman Ramadan Abedi, was working as part of a terrorist group.
Speaking from Downing Street after an emergency Cobra meeting, May said the investigations of the security services and police have “revealed it is a possibility we cannot ignore that there is a wider group of individuals linked to this incident”.
The prime minister said: “The police have asked for authorisation from the secretary of state for defence to deploy a number of military personnel in support of their armed officers.”
This means some armed police officers guarding events will be replaced by soldiers in an operation codenamed Temperer. It is the first time that the operation, which was first revealed in 2015, has been put into effect. The plan is believed to allow up to 3,800 troops to be deployed in support of the police, replacing armed officers at many sites to free them up for patrols in key areas.
Police and security services are urgently seeking to establish if the Manchester suicide bomber, who killed at least 22 people including several children, was acting on his own or was linked to a wider organisation.
The killer was named by Greater Manchester police as Salman Abedi, after his identity was first revealed to reporters by US security sources. Abedi, a 22-year old Mancunian of Libyan heritage, was known to the police and security services although was considered a peripheral figure. Media reports on Wednesday sad he had just returned from the north African country.
Greater Manchester police chief constable, Ian Hopkins, said: “The priority remains to establish whether he was acting alone or as part of a network.”
The investigation was understood to be focusing on whether Abedi was directly supported by other conspirators or built the bomb himself to instructions obtained over the internet. The device he used was described as homemade and crude although it was stable enough to be transported, and exploded with devastating effect. It was believed to have been constructed in Britain.
Abedi detonated the bomb in the foyer of the Manchester Arena on Monday night as thousands of children and their parents streamed out at the end of a concert by US pop star Ariana Grande, sending bolts and shrapnel flying into victims.
The death toll from the attack rose from 19 to 22 on Tuesday in what was the worst terrorist attack in Britain since the 7/7 attacks on London in 2005. Two girls aged eight and 18 and a 26-year-old man were the first to be named among the victims, and 59 people were hospitalised, several with life-threatening injuries including a number of teenagers. Two more victims, 15-year-old Olivia Campbell from Bury and Kelly Brewster, 32, were named in the early hours of Wednesday
The youngest victim of Abedi’s attack was Saffie Rose Roussos, eight, from near Preston. She had been to the concert with her mother, Lisa, and sister Ashlee who were both hospitalised. Her school, Tarleton community primary, described her as “a beautiful little girl in every aspect of the word”.
May chaired a two-hour meeting of the cabinet security committee, Cobra, in the morning and attacked the “appalling sickening cowardice” of the bomber in a Downing Street address. “We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish but an opportunity for carnage,” May said. Campaigning in the general election was suspended.
One senior security source described Abedi’s attack involving the acquisition of bomb-making materials or the construction of a viable device as “a game-changer” that has “rocked us backwards”, because a successful bomb plot has not been seen in the UK since the 7/7 attacks in 2005.
Investigators have gathered CCTV footage showing Abedi approaching the foyer in in the MEN Arena, which conclusively shows the explosion was deliberate and caused by a suicide device. He was also carrying ID on him when he detonated the bomb.
In the aftermath, police raided Abedi’s house in the Fallowfield area of south Manchester and carried out a controlled explosion, apparently blasting open the front door. A 23-year old man was also arrested in nearby Whalley Range, where Abdei spent some of his childhood. Armed officers also searched a block of flats in the area.
An Isis-related website claimed the attacker was “a caliphate soldier” who had killed “crusaders”, although it remains unclear what links if any Abedi had to the terror group.
A second dead victim was Georgina Callender, 18, an Ariana Grande “superfan” who met the singer in 2015. She was on the second year of a health and social care course at Runshaw College in Leyland, Lancashire. Her former school, Bishop Rawstorne Church of England academy, described her as a “lovely, popular” student.
Friends of a third victim who was killed, John Atkinson, 26 and from Bury, paid tribute to “an amazing young man so kind and thoughtful”. At least 15 people were also reported missing on social media by families who were plunged into what May, described as “unimaginable worry”.
The Queen expressed her “deepest sympathy” to all those affected and said “the whole nation has been shocked by the death and injury”. The Labour leader Jeremy Corbyn warned against allowing “communities to be divided by this kind of appalling, atrocious act of violence”.
Condolences flowed in from world leaders including Vladimir Putin and Donald Trump, who described the attacker as “a loser”.
On Tuesday evening, thousands of people filled Albert Square in front of Manchester town hall for a vigil. The home secretary, Amber Rudd, joined Andy Burnham, Greater Manchester’s recently elected mayor, and Corbyn on stage where the lord mayor of Manchester, Eddy Newman, said the vigil was a chance to “express solidarity” with the victims.