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It came hours after officers arrested an 18-year-old man believed to have planted the failed device.

Last night police were also searching for a “woman dressed in a niqab” suspected of masterminding the Parsons Green Tube attack.

A source said she was wearing women’s Islamic dress and carried a Lidl shopping bag containing the bucket bomb.

It was left at a drop-off point and later picked up by another terrorist, who then left it on the train.

The “female” suspect is said to be part of a network of at least four people thought to be behind Friday’s explosion.

Police were last night racing to catch them before they strike again.

Yesterday an 18-year-old was arrested in Dover amid claims he was trying to flee the country.

Sources close to the investigation suggest he was known to counter-terror officers and was a supporter of Islamic State.

Hours later officers raided the home of elderly couple Ronald Jones, 89, and wife Penelope, 71, in Sunbury-on-Thames, Surrey.

The couple have fostered hundreds of children and were given MBEs in the 2009 New Year’s honours list for services to children and families.

They were being questioned last night but are not under arrest.

Some of the youngsters they have looked after were believed to be refugees from Iraq, Eritrea, Syria, Albania and Afghanistan.

Neighbours said they had recently been caring for a “problematic foster child” aged 18.

Serena Barber, 47, who has known them all her life and lives in a property backing on to their home, said: “All I know is that they have two boys at the moment, both are foreign.

“One is very quiet and polite, the other who is 18, is awful. He kept getting in trouble with the police and they didn’t know what to do with him.”

The couple are understood to have been helping Surrey council with its Syrian vulnerable persons relocation programme.

The council started the scheme after the government pledged to take in 20,000 more Syrian refugees in response to the migrant crisis.

Police sources said the female suspect in the Tube attack was captured on CCTV wearing a niqab – a veil covering the head and lower face.

Officers are probing whether it may have been worn by a man to disguise his identity.

The source said: “One of the three directly linked to Friday’s bomb appears to be a woman, dressed in black with her head covered in a niqab and wearing glasses.

(Image: TWITTER)

"She has a similar bag in her hand. We understand the men wore glasses and baseball hats, presumably to avoid CCTV. We have names and the hunt is on to dig them out.

“Someone is hiding these people in a safe house, we need to get to them before they strike again.”

The arrest came a day after the blast on a packed District Line train in Parsons Green station, West London, injuring 29.

The device could have killed dozens but failed to detonate properly.

(Image: REUTERS)

The teen arrested at Dover was detained on suspicion of preparing a terrorist act, sparking an evacuation of the ferry terminal.

He was taken to London to be interrogated by Joint Terrorism Analysis Centre officers under supervision of the police Counter Terror Command.

Our source said: “They have 600 lines of inquiry across the UK and believe the Parsons Green incident was planned by a group who came into London and then dispersed into separate safe houses in the Greater London area.”

Home Secretary Amber Rudd branded the arrest “very significant” but stressed that the manhunt was “ongoing”.

(Image: EPA)

Thousands of armed police patrolled UK cities yesterday as the terror threat level remained “critical”.

If the female suspect linked to the planned atrocity is caught, she will join the ranks of other infamous women terrorists, including “White Widow” Samantha Lewthwaite and Isis moll Sally Jones.

Muslim convert Lewthwaite, 33, married 7/7 suicide bomber Germaine Lindsay before fleeing to Africa to join extremist group Al-Shabaab.

Kent-born Jones, 50, sneaked into Syria to join Isis with her terrorist husband Junaid Hussain, who was killed by a drone strike in 2015.