Dr Herrington said the attack instead bore echoes of the case of Damon Smith, 20, who was jailed for 15 years in May after leaving a home-made bomb in a rucksack containing ball bearing shrapnel on a Jubilee Line train.

Smith, who had autism and was not motivated by terrorism, created the bomb after finding a guide online.

Scotland Yard said the area surrounding Parsons Green Tube station has been evacuated so specialist officers could “secure the remnants of the improvised device and ensure it is stable”.

A spokesman added: “A small number of residents will be affected by the evacuation, which encompasses an area approximately 50 metres in radius.” The Tube station remains closed.

Meanwhile, more than 50 residents evacuated following the attack are being looked after at the Lancaster Court community centre, reports Nicola Harley.

Council officers said they do not know how long people will be stuck here.

Asked about the London terror attack by reporters outside the White House in Washington, US President Donald Trump said: “It’s a terrible thing. It just keeps going and going, and we have to be very smart, we have to be very, very tough. Perhaps we are not nearly tough enough.

“It’s just an absolutely terrible thing. In fact, I’m going to call the Prime Minister right now. We have to be tougher and we have to be smarter.”

The security services have identified a suspect in the Parsons Green bombing from CCTV footage, according to an unconfirmed report by Sky News.

Passenger Emma Stevie was in the Tube carriage when the device exploded. Here she describes the moment people were crushed as they tried to escape:

Theresa May has rebuked US President Donald Trump over his Parsons Green comments on Twitter, saying: “I never think it’s helpful for anybody to speculate on what is an ongoing investigation.”

The Prime Minister also condemned the “cowardly” attack,  saying: “This was a device intended to cause significant harm.”She also confirmed that the UK terror threat level remains unchanged at severe and is not being raised to critical.

Mrs May was speaking following a meeting of the Government’s emergency Cobra committee.

Scotland Yard has dismissed a claim by Donald Trump that the Parsons Green Tube bombers had been “in the sights” of police as “pure speculation”.

A spokesman for the Metropolitan Police said the US president’s comments – which did not correspond with any information released by the UK authorities – were “unhelpful”.

Mr Trump made the provocative allegation in a trademark tweet as police launched a major manhunt to track down the perpetrators responsible for planting the device on a packed commuter train.

“Another attack in London by a loser terrorist,” he wrote. “These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!”

His claim threatened to spark a fresh row over US-UK intelligence sharing arrangements which have already come under strain since he entered the White House.

A Scotland Yard spokesman said: “The comments are unhelpful and pure speculation. If anyone has got any evidence or information, please contact the anti-terrorism hotline.”

Tube passenger Ryan Barnett said there was a crush of people on the stairs as commuters ran from the scene of the blast.

The 25-year-old, who works in politics and had been travelling to his office, said: “The stewards and other passengers, they are shouting ‘stop, stop, stop’, so I ended up squashed on the staircase, people were falling over, people fainting, crying, there were little kids clinging on to the back of me.

“It is absolute chaos, it was quite scary because at one stage we thought we might be trapped there – I heard a pregnant woman lost her shoes and had fallen over.”

The blast comes with the UK on high alert following four terrorist attacks so far this year.

Authorities have foiled 19 plots since the middle of 2013 – including six since the Westminster atrocity in March.

The incident comes a day after figures revealed terror-related arrests in Britain have hit a new record high, with suspects held at a rate of more than one every day.

Police and MI5 are running 500 investigations involving 3,000 individuals at any one time, while there are also 20,000 former “subjects of interest” whose risk must be kept under review

Tube passenger Lauren Hubbard, who was end in the end carriage of the train with her boyfriend, said she heard a loud bang and saw a “fireball” coming towards her, reports Laura Hughes.

The 24-year-old said: “It was hot and just came towards you, this flaming orange coming towards you. It smelt like burning.

“We hid behind cement boxes on the tube platform and were the last people to get off the platform.”

She said her first thought was: “Terrorist attack… I’m going to die.”

Another witness, who works next to the station, said he saw people pouring out the station looking “shellshocked” and “panicked”.

Another witness told the BBC: “I was walking towards the station and suddenly lots of people came running down the stairs shocked and tearful and saying that there had been an explosion.

“People were saying there had been a wall of flames coming down the Tube carriage. I turned around and came back from the Tube station with all of them.”

Scotland Yard have confirmed that no arrests have so far been made.

A spokesman said: “Detectives from the Counter Terrorism Command, part of the national counter-terrorism police network, are making fast-time inquiries to establish who was responsible and are working closely with the security services. So far there have been no arrests.”

Passenger Jose Sarango was on a Tube train about to pull into Parsons Green as terrified commuters began along the tracks opposite to flee the device, reports Steve Bird.

“We had nearly reached Parsons Green at about 8.25am when our train from Victoria suddenly stopped. The driver announced that there was a problem with the train ahead.

“That was when we saw people, mainly adults and many wearing suits, filing nervously along the train tracks opposite away from the station towards Fulham.

“Many of them looked panicked as they tried to pass trains stacked up. Some of them were running and looking behind at the train they had just been on in the station. They were clearly very worried.”

Mr Sarango, a 52-year-old cleaner from Stockwell who was travelling to Parsons Green to have breakfast with his wife before work, said he was stuck on his train for an hour and watched as scores of passengers fled past his carriage.

“The driver then told the passengers to move to the back and we were let off to walk along the tracks,” he continued.

“I felt very nervous. No one knew exactly what was happening. People had begun looking at their phones and saw reports of people saying there had been some kind of explosion. That news meant we were even more worried.”

Mr Sarango and his fellow passengers were led down a back route along the tracks to Parsons Green.

“I was reassured to see lots of armed police, police cars and sniffer dogs. But, it was scarey.”

His wife, Cecelia Velez, had been waiting to have breakfast at Gail’s coffee shop on Fulham Road before their shift as cleaners at a local shop.

When she called him he warned her about the terror fears. Minutes later, police ordered her and staff at the restaurant to take cover downstairs.

“I suffer a blood pressure condition. And, I could feel my blood pressure drop,” she said. “We didn’t know what was going on, we were simply told that there had been an ‘incident’, but our phones told us otherwise.

“I still feel a little cold, but the staff here have given me coffee and sugar to help.”

Home Secretary Amber Rudd has condemned the Parsons Green blast, saying:  “Once more people going about their everyday lives have been targeted in a callous and indiscriminate way.

“My thoughts are with all those injured and affected. The police and emergency services were quickly on the scene and I would like to pay tribute again to their professionalism and tireless commitment.

“I am receiving regular updates on what is an ongoing and active investigation. I would urge Londoners to remain alert and assist the police and emergency services as much as they can.”

The mayor of London, Sadiq Khan, has issued a statement in which he said: “This was an attempt by evil and cowardly individuals to kill and disrupt our way of life.”

Mr Khan told LBC: “There is a manhunt under way as we speak.”

Sadiq Khan: Blast shows threat facing London

Residents surrounding the Tube station have been evacuated from homes as police check  for any other potential devices, reports Nicola Harley.

“There are searching homes, workmen told me they are checking for any other devices,” a witness said. “We don’t know when they will let us return.”

Witness Luke Warmsley was in the carriage where the device exploded.

“It was like a large match had gone off at the end of the carriage,” he told LBC radio.

“There was a plume of smoke that went off. I looked down the carriage and just saw more and more people running towards me.”

Another witness said she saw people with burns and medics were placing plastic over their injuries.

Stephen Cowan, the leader of Hammersmith and Fulham Council, the area in which the attack took place, said he was “outraged” by the explosion. He said:

“Around 8.20 this morning a package exploded in the final carriage of a District Line train at Parsons Green station, it is being treated as a terrorist incident.

“What is extremely impressive was that the emergency services were on to the situation immediately, there is a completely co-ordinated effort and there is no reason to be overly concerned.

“I would stress that people should be reassured because this is being dealt with very effectively by everyone in the emergency services.

“I can’t tell you how angry I am, I am absolutely outraged that someone could do this – attacking innocent lives, putting those people at risk and causing fear and harm is not a way to express yourself.

“This kind of terrorism needs to be stamped out, but people need to recognise that in London we have one of the most amazing cities anywhere, they are not going to damage us, they are not going to beat us.”

In addition to the 18 victims taken to hospital by ambulance, four others took themselves to hospital, the NHS said. Here is where they are being treated:

• Imperial – three patients
• Chelsea and Westminster – eight
• Guy’s and St Thomas’ – eight
• Central London Community Healthcare Urgent Care Centre in Parsons Green – three

12:09pm

Jeremy Corbyn thanks emergency services for response

Writing on Twitter, Labour leader Jeremy Corbyn has thanked the emergency services for their response to the terror attack:

Donald Trump denounces attack by ‘loser terrorist’

US President Donald Trump has denounced the attack by a “loser terrorist”, suggesting that such attackers “were in the sights of Scotland Yard”:

Police: Blast caused by ‘detonation of improvised explosive device’

Assistant Commissioner Mark Rowley said the Parsons Green explosion was caused by the “detonation of an improvised explosive device”.

Mr Rowley said it was understood that “most” of the 18 injured were suffering from “flash burns”.

• He added that:
• Hundreds of detectives are involved in the investigation – including scouring CCTV footage and speaking to witnesses
• MI5 is supporting the police investigation
• Londoners can expect to see an enhanced police presence on the capital’s transport network this weekend

‘Remain vigilant’: Assistant Commissioner Mark Rowley warns the public

One witness told The Telegraph’s Laura Hughes: “It was extraordinary – just panic as everyone legged it off the train.

“I got stuck in a mass pile-up as everyone tried to get down the stairs. People were falling on top of each other trying to get out.”

A passenger on the train ahead, who gave his name only as Mike, told the Telegraph: “We were taken off the train at Fulham Broadway. The driver reported a mayday call had been received from Parsons Green.”