Organisers of the annual Oxford-Cambridge boat races will decide later whether the event will take place on Sunday after an unexploded Second World War bomb was found near the starting line.
Police were called out after a member of the public spotted the device near Putney Bridge, just yards from where the annual rowing event will get under way.
A Scotland Yard spokesman said: "Police were called by a member of the public at approximately 1.50pm on Saturday reporting what they thought to be World War Two ordnance on the Chelsea shoreline by Putney Bridge.
He continued: "Officers including the Marine Policing Unit attended. The item is currently submerged."
Marine experts examined the submerged bomb and the races are expected to go ahead as planned.
Race director Michelle Dite said on Saturday: "Any decisions regarding changes to the event will be made in conjunction with the police."
Thousands of people are expected to line the banks of the Thames to watch the 163rd edition of the Boat Race between the two universities.
The men's event starts at 5.35pm, while the women's race - in its 72nd year - begins at 4.35pm.