Paolo Nespoli-Eustace, 18, was last seen on CCTV arriving at London Kings Cross from Peterborough after 1am on Wednesday, October 30.
Paolo has been suffering with mental health issues and was recently sectioned for his own safety, but escaped from the facility where he was being cared for in Peterborough.
He was seen on CCTV boarding a train bound for London just after midnight and was recorded arriving at Kings Cross at 1.16am on the morning of October 30.
There have been no confirmed sightings of Paolo, who has links to both cities, since then and his family are desperate for information.
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He has no history of violence but given his condition and the length of his disappearance, he is likely not of sound mind and should not be approached; any sightings should be reported to the police by ringing 999 and quoting incident 533 of October 29.