An 18-year-old man has been arrested in Dover in connection with attack on a London Underground train in Parsons Green.
The suspect was arrested by Kent Police in the port area under the Terrorism Act on Saturday morning.
He remains in custody at a local police station and will be transferred to London in due course.
The Metropolitan Police described the arrest as “significant” but did not give any further information on the man’s alleged role, having previously suggested investigators were hunting multiple suspects.
Deputy Assistant Commissioner Neil Basu, the senior national co-ordinator for counter-terrorism policing, said: “We have made a significant arrest in our investigation this morning. Although we are pleased with the progress made, this investigation continues and the threat level remains at critical.
“The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation. We are not, at this time, changing our protective security measures and the steps taken to free up extra armed officers remain in place.
“This arrest will lead to more activity from our officers. For strong investigative reasons we will not give any more details on the man we arrested at this stage.”
Police are continuing to appeal for information on the attempted bombing, which struck a packed District Line train at the height of Friday morning’s rush hour.
Commuters described a “fireball” sweeping through the carriage, injuring 29 people and prompting a dangerous stampede by those trying to flee.
Detectives have so far spoken to 45 witnesses are receiving photos and videos from the public, while scouring CCTV footage.
Theresa May raised the terror threat level to “critical” overnight, signalling that a new attack could be imminent as armed police and soldiers were deployed to patrol potential targets.
Assistant Commissioner Mark Rowley, the UK’s most senior counter-terror officer, previously said police were “chasing down suspects”.
“There is no doubt in my mind that those responsible intended to cause great harm and injury,” he added.
The security minister, Ben Wallace, said there were additional fears that the bombing could spark copycat atrocities, with previous terror attacks thought to have pushed extremists into action.
He told BBC Radio 4’s Today programme the Joint Terrorism Analysis Centre raised the threat to “critical” because of intelligence, extremists’ behaviour and the “pace of the investigation”.
“In this case imminent means that we know there was an attempted attack on a Tube yesterday morning, it used the type of explosive similar to that used in Manchester [but] it didn’t go off,” Mr Wallace added.
“We are trying to track down who did it, whether it’s a bomber or bombers…there’s potentially a very dangerous individual or individuals out there and we need to track them down.”
Like the bomb detonated by Salman Abedi at Manchester Arena, the Parsons Green device contained triacetone triperoxide (TATP) and nails.
The sensitive explosive, which can be made in under a day using chemicals widely available on the high street, has become a hallmark of Isis attacks including those in Paris and Brussels, as well as numerous plots across Europe.
The Independent was able to access the bomb-making manual believed to have been used for the device in Parsons Green – as well as the Boston Bombings and other massacres – in under five minutes on Saturday morning.
Both al-Qaeda and Isis have details instructions on making improvised explosive devices, with forensics experts working to establish the origin of the latest bomb.
Parsons Green explosion – in pictures
Experts believe the device in Parsons Green used TATP, a “crude” delay timer and fairy lights, but failed to fully explode.
Commuters described a “fireball” bursting from the device and a strong chemical smell, with wires and fairy lights seen trailing from the flaming plastic bucket.
David Videcette, a former counter-terrorism detective in the Metropolitan Police, said: “The first fireball normally intended to set off the main charge, which is the explosives.
“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.”
In a similar incident at Brussels Central Station in June, a suitcase bomb packed with nails burst into flames but did not explode, while Stockholm attacker who killed five people with a lorry in April was burned by a failed device.
In October, a homemade bomb made using an al-Qaeda manual failed to detonate on a Jubilee Line train at North Greenwich Tube station.
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.
Recently Isis has issued advice on how to make bombs using the explosive TATP and incendiary devices, while calling for indiscriminate attacks in countries including that UK that are targeting its militants in Syria and Iraq.
In a claim of responsibility issued on Friday night, the terrorist group said the Parsons Green attack was carried out by the “soldiers of the caliphate”, adding: “What is coming is more devastating and more bitter, Allah willing.”