Poppy Bourke was reported missing last week but was back in Sussex on Friday night.
However, she has been reported missing again after leaving her home at 7.25am this morning (Sunday) and getting on a train from Angmering train station, according to Sussex Police.
Officers believe Poppy is in the Kings Cross or Camden areas of London or may be travelling to Hertfordshire, where she was found on Friday.
Poppy is described by police as white, 4ft 9ins, of slim build and with very long straight black hair. She was last seen wearing blue jeans and a green jacket.
Anyone in that area of London or in Hertfordshire is asked to report should they see her. Reports can be taken online quoting serial 272 of 11/02.