Police have released a photo of a "runner" who was found collapsed in the street, in an effort to trace his family by Christmas.
The man, who is thought to be in his 60s, was discovered in Kendra Hall Road, Croydon, on Thursday having apparently suffered a heart attack.
He is in a critical condition in hospital, and police said they have been unable to identify him.
Police believe he may have been out running before he collapsed.
They said they wanted to "contact his friends and family before Christmas".
"It's very important that we identify this man as soon as possible to make his loved ones aware and to safeguard anyone he may care for," Det Supt Jane Corrigan said.
The man was wearing a white T-shirt, black tracksuit trousers and white Asics trainers and also had on him a Yale key and a watch.