Audrey Kessie Nyanor was taken from Walworth Police Station A search is continuing for a toddler who police mistakenly allowed to be taken by an impostor from a station in south-east London.
Audrey Kessie Nyanor's mother, 36, was arrested on Thursday in Southwark, south-east London, over alleged immigration offences.
A woman claiming she was a friend of Audrey's mother went to the station and took the 18-month-old girl.
The Metropolitan Police has begun an internal inquiry into what happened.
Six arrests
Police internal review body the Department for Professional Standards is reviewing the circumstances of the incident to find out what sort of description was given of the woman meant to collect the child and how it was checked.
The Met has also voluntarily referred the matter to the Independent Police Complaints Commission.
The incident happened on Thursday morning when officers were keen that Audrey should not stay at the police station.
They spoke to several friends and family members about collecting the toddler.
The woman claimed to police she was a friend of Audrey's mother A 46-year-old woman who said she was the mother's friend arrived and police let her take the girl away, but it soon became clear she was the wrong woman.
Police say they are very concerned and are trying to find Audrey.
Her mother was later released on bail.
Audrey, who is of Ghanaian descent, was last seen wearing pink trousers, a pink coat and a pink and white hat.
Police say the woman who took Audrey is also believed to be of Ghanaian heritage, medium build, with short black hair, and was last seen wearing a bright red coat and dark knee-length boots.
A Metropolitan Police spokesman said: "At approximately 10.30am a woman attended the police station purporting to be a 46-year-old friend of the mother.
'Very concerned'
"The details she gave to police corresponded with the name of the woman Audrey's mother had told police would pick up her daughter, and as a result she left the police station with the child at approximately 10.40am.
"Subsequently it became apparent that the woman was not the same person who had been organised to pick up the child.
"As a result a police investigation began to find Audrey, and to identify and trace the woman who took her.
"Whilst there is no evidence at this stage to suggest that Audrey has come to any harm, we are very concerned that she remains missing and we are making every effort to ensure she is safely returned."
Police issued images of the woman at the police station and at Elephant and Castle underground station, where she was pictured carrying Audrey.
Several places have been searched and six people have been arrested in connection with the investigation, but not the woman who collected the toddler.