Sian Blake: Former EastEnders Actress Missing
The 43-year-old, who has not been seen for almost three weeks, is reportedly very frail and suffering from motor neurone disease.
She was last seen on Sunday 13 December in Waltham Forest, east London, with eight-year-old Zachary and four-year-old Amon.
Officers believe they are still in the London area - but Ms Blake also has links to Cambridge and Colchester.
The force added: "Officers are increasingly concerned for Sian and the children's wellbeing."
The actress appeared in 56 episodes of EastEnders between 1996 and 1997, according to the IMDb website.
She is described as being black, 5ft 2in and slim with short black hair.
She wears glasses but can also wear contact lenses.
:: Anyone with information should call police on 101 or Missing People on 116000, quoting reference 15MIS050346.