Sophie O'Dere, from Morecambe, was last seen boarding a train to London at Manchester Piccadilly railway station at 5.55pm on Monday, July 13.
CCTV footage of Sophie later shows her leaving Euston railway station in Camden, but police believe she may have travelled to Wales or closer to home since then.
Officers are becoming increasingly concerned for the 16-year-old's welfare and have launched an urgent appeal to help find her.
A spokesperson for Lancashire Police, said: "We are becoming increasingly concerned for Sophie’s welfare and I would encourage anybody who sees her or knows where she is to contact us immediately.
"We now believe that Sophie may have left London and potentially travelled to Wales or even be back closer to home.
"I’d ask anyone who knows anything at all about her whereabouts to please get in touch. I would also appeal directly to Sophie to make contact with us if she sees this appeal."
Sophie - who has links to Leyland, London and Llandudno in Wales - is described as 5ft 10in tall, of medium build, with brown eyes.
She was last seen wearing a pale blue denim jacket, a turquoise patterned top, a mustard colour knee length skirt, grey striped socks and dark coloured shoes with white soles.
Anyone with information about Sophie’s has been urged to contact police on 101, quoting log 1068 of July 13.