Veronica Sbircea, aged 15, left her home in Kettering on the evening of Thursday July 21.
She was seen in Ponders End, Enfield, on Wednesday July 27 and then in Brixton in and around the KFC store on Brixton Road, London, on Thursday, July 28. There are no confirmed sightings of Veronica since then.
Officers believe she may now be in Birmingham, although there is a possibility that she remains in London.
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Veronica is white, 5ft, with long straight black hair and brown eyes. She was last seen wearing dark leggings and a black top.
She is originally from the London area and may have possibly travelled back there.
Veronica is known to approach people to use their mobile phone temporarily, and officers are urging anyone who may have been approached by her or have seen her, to make contact.
Officers are urging Veronica, or anyone who may know of her whereabouts, to contact Northamptonshire Police on 101.