Police are warning that a woman is handing bottles feared to contain bleach to babies in pushchairs.
Officers in Greenwich said that the white woman. aged in her 50s, was seen in Eltham High Street, south east London, giving youngsters the bottles.
A baby boy was taken to hospital after being taken ill.
A spokeswoman for London Ambulance Service said: "Our staff treated a baby boy who had been taken unwell, who was taken to Queen Elizabeth Hospital, Woolwich."
The Metropolitan Police alerted the public to the danger via Twitter. In a message from @MPSGreenwich, police said: "WARNING! Last 1/2 hr. #Eltham High St. White female in 50s giving children in pushchairs bottles containing believed bleach."