CCTV images of teenager Shelley Pratt have been released to help find the missing 14-year-old.
The CCTV stills show the last confirmed sighting of Shelley in Croydon, who disappeared on November 8 after school.
The stills show 14-year-old Shelley at George Street tram stop in Croydon Town Centre at approximately 9.30pm.
It is not known if Shelley, who was on her own, boarded a tram or where she was heading towards. She is seen wearing her school uniform trousers and a distinctive dark purple hooded jumper with the slogan "I love (heartsymbol) London."
Earlier that day, Shelley was seen at 3.30pm after she got off a route 196 bus on Portland Road outside the Harris Academy. Shelley then started to walk south down Portland Road in the company of one other girl and a boy.
Shelley is described as white and around 5'6" or 5'7" inches tall with long brown hair.
Detective Chief Inspector Mark Dunne said: "It has now been three-and-half weeks since Shelley went missing and concerns for her safety are understandably increasing by the day.
"Shelley has had absolutely no contact with friends or family since her disappearance. It is possible that Shelley may be with someone, perhaps in a house or a flat. I am encouraging people, particularly Croydon residents, to be 'nosey neighbours' and be extra vigilant as to the movements of any young people who are new to the area.
"I would also encourage people to look at her picture closely. This picture can be found on the Met police and Missing People websites. I urge anyone who may have seen Shelley since 8 November to contact us."
Anyone with information about Shelley's whereabouts is urged to call the incident room on 020 8721 4906.