Bedfordshire Police are appealing for any information about Milly, who has been missing since May 24.
A Bedfordshire Police spokeswoman said: “Have you seen missing 15-year-old Milly?
“Milly has been missing from Leighton Buzzard since May 24 and we are growing increasingly concerned for her welfare.
“Milly is of slim build and has dyed blonde/orange hair in a bun. She was last seen wearing a black and gold jacket with black bottoms.
“If you have seen Milly or have any info on her whereabouts please call 101 or report online.”