A suspected unexploded World War Two bomb discovered on the banks of the River Thames could threaten the university Boat Race.
The find was made near Putney Bridge, west London, a day before the 163rd Oxford v Cambridge race.
No decision will be made until Sunday on possible “changes to the event”.
The device is submerged and a bomb disposal unit will have to wait until the tide goes out in order to take a better look at it, police said.
The Thames river tide receded at about 01:00 BST. Organisers will meet police later on Sunday to decide if the race will be affected.
The Met said it was too early to say whether the race would be called off.
Crowds of up to 300,000 people are expected if it goes ahead.
Spectators have been warned to be vigilant following the attack at Westminster, reported the Telegraph.
“The security threat level remains at severe and, as always, we would encourage those attending to be aware of their surroundings and be vigilant,” the Met said in a statement.
Roads in the area where the suspected bomb is currently remain open.
The Met confirmed officers were in attendance on the northern bank of the river at Chelsea.
A statement said: “Police were called by a member of the public at approximately 13:50 BST on Saturday… reporting what they thought to be World War Two ordnance on the Chelsea shoreline by Putney Bridge.
“Officers including the Marine Policing Unit are in attendance.”
Michelle Dite, race director of the Boat Race, said: “At this stage the races will go ahead as planned.
“Any decisions regarding changes to the event will be made in conjunction with the police. No decision will be made today.”
The BBC understands the bomb was found by a passing sailor and was located on the Chelsea bank of the river on the north side of Putney Bridge.
The races start on the other side of Putney Bridge, with the women’s race at 16:35 BST and the men’s race at 17:35 BST.