He alerted police who, searching the area, found a second foot in a matching shoe by the river in Mericourt, some 70 kilometres northwest of Paris, a police source said.

Investigators have been unable so far to identify the person.

The boot size is a European 43 (9.5 UK, 10 US).

Prosecutors said the launch of an official homicide investigation allowed them to deploy the widest range of investigative tools to elucidate the case. There was not, they cautioned, any certainty that a murder had indeed taken place.