Essex mum Jamie-Leigh Kelly, 31, left a centre for vulnerable mothers and children in Colindale in north London on 19 September with her three-year-old daughter and newborn baby boy, against the wishes of staff.

The Metropolitan Police said she left the centre at around 7.40pm, putting them in a blue Ford Fiesta which then drove away "at speed".

Officers added it is believed the driver is a woman, and has yet to be identified, and the car has not been found.

The woman has links to Thurrock in Essex and Havering in east London, police said.

On Wednesday a man in his 50s was arrested in connection with the investigation, on suspicion of child abduction and conspiracy to kidnap. He remains in custody in Essex.

Detective Superintendent Lewis Basford, 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 as he is new-born, so please put him first and contact us."

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The detective added: "I have a team of detectives carrying out extensive lines of enquiry 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."

Police said there is no indication the child has been harmed but officers are "extremely concerned" about their welfare.

Ms Kelly is described as white with green eyes, five feet four inches tall, and was wearing a white jumper or long-sleeved top, white jogging bottoms and white trainers at the time of her disappearance.