An ice cream tub containing a substance believed to be human placenta has been found by a dog walker, police say.
The tub was found on a footpath near Tooting Bec Athletics Track, south London, at about 06:55 GMT on Saturday, Scotland Yard said.
Medical staff who examined photographs suggest it is human placenta which could be two weeks old.
Police are now concerned that a vulnerable woman has given birth and may require medical assistance.
The substance has been taken for pathological examination.
Police believe the tub was partially buried before being unearthed by an animal. Specialist police "blood" dogs searched the area but did not find any other items.
Chief Inspector Nick Aldworth said: "Obviously we are at the very early stages of the investigation into what has been found, but the suggestion that it could be a human placenta does give us concern that there could be an individual whose health and wellbeing may be at risk.
"That said, we retain an open mind regarding the item and how it came to be buried on the pathway pending the results of testing and any confirmed information coming to light.
"Therefore we are appealing to the person who buried the item to contact us, or anyone who knows the person who buried the item. Your call will be treated in the strictest of confidence."
Police are also appealing to anyone who uses the path at the back of Tooting Bec Athletics Track to contact them if they have recently seen anything there which was out of the ordinary.