A second man has been arrested by detectives investigating the terrorist attack at Parsons Green Tube station, the Metropolitan Police has said.

The 21-year-old man was arrested in Hounslow at around 11.50pm on Saturday under the Terrorism Act. He was taken to a south London police station and remains in custody.

Meanwhile, new footage, obtained by ITV News, shows a person carrying a large Lidl bag on the morning of the attack. The CCTV footage was reportedly filmed near one of the suspect's homes in Sunbury-on-Thames in Surrey, around 90 minutes before the explosion injured 30 people.

A spokesman for New Scotland Yard said: "We are aware of the footage. This is a live investigation and we will provide further updates as it progresses. Not prepared to discuss further at this time."

Armed police search home linked to teenage Parsons Green Tube bombing suspect in Surrey

Forensic officers carried out a search of a property in Stanwell, Surrey, close to Heathrow Airport.

Following the two arrests, the Joint Terrorism Analysis Centre (JTAC) lowered the UK's terror threat level from critical to severe, meaning an attack is thought to be highly likely but not imminent.

After the attack on Friday morning, photos circulated online of the partially exploded device, which had wires protruding from a flaming bucket.

Assistant Commissioner Mark Rowley, the country's top counter-terrorism officer, said: “We are getting a greater understanding of the preparation of the device.

"There is still much more to do but this greater clarity and this progress has led JTAC - the independent body that assesses threat - to come to the judgement that an attack is no longer imminent."

An 18-year-old man was detained in the departure area of the port of Dover in connection with the attack on Saturday morning.

He was arrested by Kent Police under the Terrorism Act and remains in custody at a London police station.

Deputy Assistant Commissioner Neil Basu described the arrest as “very significant”.

A search was also carried out at a house in Sunbury-on-Thames, Surrey, at the home of foster care couple Penelope and Ronald Jones, who previously received MBEs for services to children and families.

The couple are said to be staying with friends for at least the next five days following the police raids, during which surrounding houses were evacuated by counter-terror officers, with residents told they had “one minute” to flee their homes.

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

Friends said the couple – who have been foster parents for almost 40 years and have taken in up to 300 children, including eight refugees – had an 18-year-old and a 22-year-old staying with them recently.

The latest arrest comes after Mr Basu said police were keeping an “open mind” over whether more than one suspect was involved in the bombing.

Thirty people were injured when the improvised device exploded during rush hour at Parsons Green station.

All but one of those injured in the explosion have been discharged from hospital, NHS England said.

Police have since identified 121 witnesses, of whom 100 have been spoken to, Mr Basu said.