Theresa May last night raised the terror threat level to critical as police scrambled to identify the attackers who tried to blow up a packed Tube train during the morning rush hour, in the fourth assault to strike London this year.

Detectives have used CCTV images from the station and train to single out the suspect believed to have planted the homemade device, but were still working to identify the bomber.

Raising the terror status to its highest level, which means another attack is considered imminent, the Prime Minister said military personnel would replace armed police “on guard duties at certain protected sites which are not accessible to the public”.

She said: “The public will see more armed police on the transport network and on our streets, providing extra protection. This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses.”

Investigators fear the bomb, which left 29 passengers requiring hospital treatment after partially exploding at Parsons Green station, had the potential to cause huge devastation if fully detonated.

Isis last night claimed responsibility for the attack, in a common tactic in the wake of Western terror attacks.

In a statement a few hours later, Assistant Commissioner Mark Rowley, the UK's most senior anti-terror police officer, suggested there may have been more than one person involved, referring to one of those behind the attack as "him" and saying police were "chasing down suspects".

He said the force was only aware of one device, and the remnants of that device are being examined by experts.

Terror attack at Parson's Green – in pictures Show all 23 1 /23 Terror attack at Parson's Green – in pictures Terror attack at Parson's Green – in pictures Parsons Green explosion A bucket with flames and wires coming out of it was photographed in the carriage after the explosion – apparently the source of the blast AFP Terror attack at Parson's Green – in pictures Parsons Green explosion An explosion on a packed Tube train has injured a number of people in west London AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion The blast triggered a stampede as commuters panicked AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion Click through for more pictures from the scene Reuters Terror attack at Parson's Green – in pictures Parsons Green explosioN A police officer and a sniffer dog stand in the street near Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Kevin Coombs Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion Handout photo issued by James Treen of emergency services attending an incident at Parsons Green station in west London amid reports of an explosion. PRESS ASSOCIATION Photo. Issue date: Friday September 15, 2017. See PA story TRANSPORT Explosion. Photo credit should read: James Treen/PA Wire NOTE TO EDITORS: This handout photo may only be used in for editorial reporting purposes for the contemporaneous illustration of events, things or the people in the image or facts mentioned in the caption. Reuse of the picture may require further permission from the copyright holder. PA Terror attack at Parson's Green – in pictures Parsons Green explosion Police vehicles line the street near Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Kevin Coombs Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion A police officer stands outside Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Kevin Coombs Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. / AFP PHOTO / Daniel LEAL-OLIVAS / ===RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / HO / @RRIGS - NO MARKETING NO ADVERTISING CAMPAIGNS NO ARCHIVES - DISTRIBUTED AS A SERVICE TO CLIENTS FROM FROM ALTERNATIVE SOURCES, THEREFORE AFP IS NOT RESPONSIBLE FOR ANY DIGITAL ALTERATIONS TO THE PICTURE'S EDITORIAL CONTENT, DATE AND LOCATION WHICH CANNOT BE INDEPENDENTLY VERIFIED == / DANIEL LEAL-OLIVAS/AFP/Getty Images AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion Emergency services attend the scene following a blast on an underground train at Parsons Green tube station in West London, Britain September 15, 2017, in this image taken from social media. TWITTER / @ASolopovas/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. NO RESALES. NO ARCHIVES. MANDATORY CREDIT Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. / AFP PHOTO / Daniel LEAL-OLIVAS / ===RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / HO / @RRIGS - NO MARKETING NO ADVERTISING CAMPAIGNS NO ARCHIVES - DISTRIBUTED AS A SERVICE TO CLIENTS FROM FROM ALTERNATIVE SOURCES, THEREFORE AFP IS NOT RESPONSIBLE FOR ANY DIGITAL ALTERATIONS TO THE PICTURE'S EDITORIAL CONTENT, DATE AND LOCATION WHICH CANNOT BE INDEPENDENTLY VERIFIED == / DANIEL LEAL-OLIVAS/AFP/Getty Images AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion Armed British police officers stand on duty outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. / AFP PHOTO / Daniel LEAL-OLIVAS / ===RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / HO / @RRIGS - NO MARKETING NO ADVERTISING CAMPAIGNS NO ARCHIVES - DISTRIBUTED AS A SERVICE TO CLIENTS FROM FROM ALTERNATIVE SOURCES, THEREFORE AFP IS NOT RESPONSIBLE FOR ANY DIGITAL ALTERATIONS TO THE PICTURE'S EDITORIAL CONTENT, DATE AND LOCATION WHICH CANNOT BE INDEPENDENTLY VERIFIED == / DANIEL LEAL-OLIVAS/AFP/Getty Images AFP/Getty Terror attack at Parson's Green – in pictures Parsons Green explosion Armed policemen stand by cordon near Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Hannah McKay Reuters Terror attack at Parson's Green – in pictures Parsons Green explosion A woman reacts outside Parsons Green tube station in London, Britain September 15, 2017. REUTERS/Kevin Coombs Reuters Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: Emergency services outside of Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Jack Taylor/Getty Images) Getty Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: A fireman walks past the police cordon at Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Jack Taylor/Getty Images) Getty Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: A tube train is seen stopped at Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Chris J Ratcliffe/Getty Images) Getty Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: Police officers stand by a cordon at Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Jack Taylor/Getty Images) Getty Terror attack at Parson's Green – in pictures Police forensics officers works alongside an underground tube train at a platform at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. "We are aware of an incident at Parsons Green tube station. Officers are in attendance," London's Metropolitan Police said on Twitter. / AFP PHOTO / Daniel LEAL-OLIVAS / (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images) AFP/Getty Terror attack at Parson's Green – in pictures LONDON, ENGLAND - SEPTEMBER 15: Emergency serivces attend to the scene near Parsons Green Underground Station on September 15, 2017 in London, England. Several people have been injured after an explosion on a tube train in south-west London. The Police are treating the incident as terrorism. (Photo by Jack Taylor/Getty Images) Getty Terror attack at Parson's Green – in pictures Members of the emergency services work outside Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. British police are treating an incident on a London Underground train on Friday as an act of terrorism, saying "a number of people" had been injured. "Terrorist incident declared at Parsons Green Underground Station," police said in a statement. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images) AFP/Getty Terror attack at Parson's Green – in pictures Armed British police officers walk through the carriage of a London underground tube carriage at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images) AFP/Getty Terror attack at Parson's Green – in pictures Members of the emergency services and armed police officers work alongside an underground tube train at a platform at Parsons Green underground tube station in west London on September 15, 2017, following an incident on an underground tube carriage at the station. Police and ambulance services said they were responding to an "incident" at Parsons Green underground station in west London on Friday, following media reports of an explosion. "We are aware of an incident at Parsons Green tube station. Officers are in attendance," London's Metropolitan Police said on Twitter. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images) AFP/Getty

The decision to deploy soldiers had freed up an additional 1,000 armed officers, he added.

Security sources say they were unaware of any specific plot or individuals involved in the incident.

Commuters described a “fireball” sweeping through their District Line carriage, burning off some victims’ hair and leaving some with severe burns.

The blast sparked a stampede of passengers attempting to flee the station, threatening to crush a pregnant woman and children caught in the panic.

Police swiftly declared a terrorist incident but have not confirmed any details of suspects after Donald Trump claimed the perpetrators were “in the sights of Scotland Yard”.

The Metropolitan Police dismissed the US President’s “speculation”, while Theresa May said the intervention was “not helpful”.

“The police and security services are working to discover the full circumstances of this cowardly attack and to identify all those responsible,” the Prime Minister added.

But British security forces will be forced once again to explain how an attack was planned and executed, using methods requiring more time and sophistication than the massacres in Westminster, London Bridge and Finsbury Park.

Images of the explosive device showed flames spurting out of a white bucket inside a Lidl bag, which appeared to contain wires and fairy lights and left a “chemical smell”.

Theresa May on Trump's Parsons Green response: "I never think it's helpful to speculate"

Sylvain Pennec, a software developer from Southfields, said: “I heard a boom and when I looked there were flames all around.

”People started to run but we were lucky to be stopping as the door started to open.”

Richard Aylmer-Hall, a 53-year-old technology consultant, said he saw people screaming and being trampled as they tried to escape.

”Suddenly there was panic, lots of people shouting, screaming, lots of screaming,” he added.

“Some people got pushed over and trampled on, I saw two women being treated by ambulance crews.”

Passenger Ryan Barnett was a few carriages down with earphones in when people started running past “screaming a mixture of 'stampede', 'attack', 'terrorist', 'explosion', 'get off the train', 'everyone run'.”

He became crushed on a staircase in the “absolute chaos”, adding: “People were falling over, people fainting, crying, there were little kids clinging on to the back of me.”

At least 29 people were injured in the blast and stampede, including victims requiring hospital treatment for severe burns.

Police said the device was being examined by forensics officers as investigators scoured CCTV and footage recorded by witnesses to identify the culprit.

David Videcette, a former counter-terrorism detective in the Metropolitan Police, said whoever launched the attack did not want to be identified.

“This person has separated themselves from the incident, hurting people without being present,” he told The Independent.

“The way this device is constructed isn’t unusual…it looks in-keeping with 7/7 and 21/7.”

Mr Videcette investigated the 7 July 2005 London bombings, which killed 52 victims on the bus and Tube before a second round of attacks two weeks later failed.

He said Friday’s bomb appeared to use a “crude” delay timer and fairy lights, which have been used to detonate previous explosive devices.

“The first fireball normally intended to set off the main charge, which is the explosives,” Mr Videcette explained.

“What often happens with people who try to make bombs is they don’t test the main charge and the initiator together.

“They know they work individually, but the difficulty comes with the chain reaction.”

The bomb used to kill 22 victims at Manchester Arena in March was more sophisticated, using Isis’ favoured explosive TATP and shrapnel packed inside a rucksack.

Parsons Green explosion: BBC's Sophie Raworth describes woman with burns "from top to toe"

Amid initial fears that a bombmaker could have manufactured other devices than the one deployed by Salman Abedi, security services raised the UK terror threat to critical.

Unconfirmed reports suggested the Parsons Green bomb was also made from TATP and had been packed with nails.

Raffaello Pantucci, the director of International Security Studies at the Royal United Services Institute, said there was no discernible pattern to the terror attacks that have hit the UK this year.

Following the Manchester bombing, attackers in Westminster, London Bridge and Finsbury Park used cars, vans and knives to attack their targets.

“If terror groups want to launch an attack they will use any means available,” Mr Pantucci said.

“The idea of a bomb that blows up and does something dramatic is still very attractive.”

He called the Parsons Green attack “pretty incompetent” but said public transport networks remain a key target for terrorist groups.

Mr Pantucci added: “They are our city’s arteries and where better to strike a random cross-section of society.”

The attack came on the same day as two separate assaults by suspected Islamist extremists in France.

Both Isis and al-Qaeda have issued online manuals for making explosive devices that have been tied to numerous attacks and plots around the world.

In October, a homemade bomb failed to detonate on a Jubilee Line train at North Greenwich Tube station.

The culprit, an 19-year-old autistic student, had used an al-Qaeda manual but a terrorist motive was not proved and he was jailed for 15 years.

Then in May, a man was found guilty of building a bomb using fairy lights and a pressure cooker with the intention of targeting a railway line in the Midlands.

Zahid Hussain, 29, filled the appliance with shrapnel and made ”improvised igniters“ from the festive decorations but the court heard a mistake in the process mean it would not have exploded.