Experts believe the bucket bomb that ignited on a packed rush-hour Tube train today could have killed dozens had it detonated properly.
The crude device, believed to have been made with the explosive TATP – known as “Mother of Satan” – is thought to have burst into flames prematurely.
But it sent a “wall of fire” through the District Line carriage, leaving 29 people injured.
A major manhunt was under way for the bombers last night as the terror threat was raised to the highest level of critical – meaning another attack may be “imminent”.
Emergency services tend to an injured woman following the blast
One expert said the bomb, which ignited at Parsons Green station, West London, was several times the size of the Manchester Arena device that killed 22 in May.
Hamish de Bretton-Gordon, ex-assistant director of intelligence surveillance for the Ministry of Defence , said: “It looks like a pretty unsophisticated device and I think we have been pretty fortunate as it doesn’t seem to have detonated.”
He added: “There was quite a big bucket, it looks like it could contain 20 litres. That can carry a lot of explosives. That size of device could have been devastating.”
Hundreds of police officers joined forces with MI5 agents in the hunt for the terrorists behind the attack.
And there were fears of another bomb or knife rampage as the culprits remained at large.
The blast came as the train pulled into Parsons Green station at 8.20am. Terrified passengers were seen covered in blood with scorched heads, legs, faces and hair as they “ran for their lives” trying to escape the horror.
The device was in a white builder’s bucket inside a Lidl foil-lined and sealable freezer bag which may have been designed to keep the highly unstable ingredients cool.
Such a mixture can ignite if it gets too warm and the heat inside the packed carriage may have caused this to happen.
Terror expert Will Geddes believes the bomb could have been primed to detonate at a different station than Parsons Green. The train is believed to have been heading to Upminster in Essex or Tower Hill, East London. This would have taken the service through busy stations in Central London, including Victoria and Westminster.
Mr Geddes said: “I think it was a premature detonation than anything else. This could have been a lot worse. They are always going to aim for maximum impact.”
Tonight Theresa May raised the threat level to “critical”. This means authorities believe a terror attack may be imminent. It is only the fourth time Britain has been put on the highest alert level during the past 11 years. Under Operation Temperer – last put in place after the Manchester attack – the Government will also deploy troops to support police.
Mrs May said: “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.
“During this period the public should go about their business in a normal way and as usual be vigilant and co-operate with the police.
“I said earlier that terrorism is a great challenge of our times but by standing together we will defeat it.” Early reports claimed the suspect had been seen on CCTV but Scotland Yard refused to comment on claims the bombers had already been identified.
The device was apparently put on the train at some point between Wimbledon and Parsons Green. No image of the suspects were released, suggesting police may know who they are.
Giving an update tonight, Met Assistant Commissioner Mark Rowley, the country’s most senior counter-terrorism officer, said: “We are making excellent progress at the moment as we pursue our lines of inquiry 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 London’s counter terrorism policing resources, assisted by colleagues from around the country and by our intelligence agency partners.
“We have hundreds of police officers trawling though CCTV footage, detectives have spoken to tens of witnesses and we have taken a large number of calls to the hotline from members of the public.”
He also said the IED had been removed and taken it away “for examination by forensic scientists”.
Mr Rowley added that up to 1,000 police officers had been freed up to provide extra safety checks and reassurance across the country.
It was the sixth terror attack so far in the UK this year and the only one in which nobody has died. There were 36 deaths in the previous atrocities. During the same period police say they have stopped six other significant plots.
NHS England said 21 victims were still being treated last night in hospital .