The child, believed to be a girl, was later found to be safe and well when her mother telephoned the hospital to say she had taken her from the Elizabeth Ward and was looking after her at home.
Staff raised the alarm at 8.45 on Monday evening after the mother, against nurses' advice, took the baby from the neonatal unit where it was under observation.
She had been with the baby during the afternoon and had later returned. Police were alerted.
Tim Battle, the Royal's director of operations, said yesterday: 'The security of the childrens' wards has been reviewed in the light of incidents in other hospitals.
'The door to the Paediatric Unit, called Garden House, is locked at night. This incident took place during visiting hours when access by parents is unrestricted.'
The mother and her baby are believed to live in the London area, but neither has been named.
A hospital spokeswoman said there was nothing to stop mothers walking into hospitals and taking their babies while wards were unlocked, unless the child was subject to an infant care order.
Such orders are placed on children who have to be kept under observation for their health.
The incident at the Royal comes just days after a new-born girl was snatched from the arms of her parents at a hospital in Nottingham.Reuse content