A second man has been arrested by detectives investigating the terrorist attack at Parsons Green station on Friday.

The 21-year-old was arrested in Hounslow at 11.50pm on Saturday, the Met’s Counter Terrorism Command said.

He was taken to a south London police station and remains in custody.

The development comes after the arrest of an 18-year-old man in the departure area of Dover ferry port on Saturday morning. The suspect remains in custody at a London police station.

On Saturday, a search was carried out at a house in Sunbury-on-Thames, Surrey – home to foster care couple Penelope and Ronald Jones, aged 71 and 88 respectively, 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.

Police officers taking part in an operation in Cavendish Road. (Jonathan Brady/PA)Police officers taking part in an operation in Cavendish Road. (Jonathan Brady/PA)

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

She described Mr and Mrs Jones as “great pillars of the community”, adding: “They do a job that not many people do.”

A key strand of the investigation has focused on CCTV as officers comb through footage to establish who planted the device, and when and where it was placed on the train.

The first suspect was arrested at around 7.50am, in the port which is the busiest ferry hub in Europe and serves as a commercial gateway to the French coast, including Calais and Dunkirk.

Tourist Daniel Vaselicu, 31, said he saw the “young and light-skinned” man being interrogated by two unarmed police officers moments before his arrest in Dover.

He told the Press Association: “He was white, not Arabic, but he wasn’t English – there was only one person there.

“Then we went to the town centre, I wanted to smoke a cigarette and have some coffee, my opinion was that he was a homeless guy and that’s why they were interrogating him.

“He was looking normal, not fighting or worried or concerned.”

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

The terror threat remains at its highest level – critical – meaning another attack may be imminent.

Deputy Assistant Commissioner Neil Basu previously said police were “keeping an open mind” about the perpetrators.

He added: “If there are other people responsible it’s our job to find them and that is part of the reason that we are remaining at critical threat.”

Home Secretary Amber Rudd said it appeared the bomber was not a lone wolf but it "too early to reach any final conclusions on that".

Asked if she could give any information on claims by Islamic State that there were other unexploded devices, she told BBC1's Andrew Marr Show: "We don't.

"It is inevitable that so-called Islamic State, or Daesh, will reach in and try and claim responsibility. We have no evidence to suggest that yet."

Mrs Rudd said £24 million of new government money was going to counter-terrorism operations across the country.

Asked if police cuts had hit anti-terror operations, the Home Secretary said: "I think it is too simple to approach it like that.

"What we have is an onslaught of attacks from Daesh trying to radicalise people."