Police are appealing to the public to help locate a mother and her two young children who have been missing since Tuesday.
Jamie-Leigh Kelly, 31, left a family assessment centre in north-west London with her three-year-old daughter and newborn baby boy against the wishes of staff.
A man in his 50s was arrested in connection with the investigation on suspicion of child abduction and conspiracy to kidnap on Wednesday. He remains in custody in Essex.
The children are under care orders and should not have been removed from the address in Colindale, which provides 24-hour social care support for vulnerable mothers and children, the Metropolitan police said.
Kelly left at about 7.40pm on Tuesday and staff at the centre were unable to prevent her from putting the children into a blue Ford Fiesta which then drove away at speed.
The Met said from its CCTV inquiries, they believe the driver was a woman. She is yet to be identified and the car has not been found.
Kelly has links to Thurrock in Essex and most recently Havering in east London.
Det Supt Lewis Basford, who leads the Met’s public protection command in the East Area, said: “We are extremely concerned about the welfare of Jamie-Leigh and her children, one of whom is less than one month old.
“Jamie-Leigh, I am making a direct appeal to you to contact us so we can ensure the children are safe and well.
“We know you have no intention for the children to come to harm, so we are asking you to please pick up the phone and have a conversation with us so we can meet with you. The baby may need medical care … please put him first and contact us on 020 7175 0793.”
Police added the baby boy was born in mid-September and, as a newborn, is extremely vulnerable.
Kelly is described as white with green eyes, of slim build and 5ft 4ins tall.
She was wearing a white jumper or long sleeved T-shirt, white jogging-style trousers and white trainers.
Basford added: “I have a team of detectives carrying out extensive lines of inquiry alongside Essex police colleagues, and both Thurrock and Havering local authorities have been extremely supportive as we continue efforts to locate the family.
“We believe Jamie-Leigh is being assisted, and we ask anyone in Essex or London or further afield who may have information to call our incident room without delay, or dial 999 with any real-time sightings.”