The '32ft' humpback whale spotted in the River Thames over the weekend has died.
Its body was reported close to the Kent shore at Greenhithe but has been floating with the tide.
A Port of London Authority pilot boat has been dispatched to find the carcass and a post mortem will be carried out.
Quickly named online as Hessy, the whale was seen between Crayford Ness and the Queen Elizabeth II Bridge in Dartford, Kent,on Sunday.
It was the first time in 10 years a humpback whale had been spotted in the Thames.
The rare species can weigh up to 30 metric tonnes in adult form.
Martin Garside, from the Port of London Authority, said on Monday: "There were a number of sightings by boats on the river during the course of yesterday.
"We have not had any confirmed sightings today.
"As far as the reports yesterday go, it is a dark-coloured whale with a small dorsal fin - so it is not anything like the beluga. This animal is darker.
"All we know at this stage is it is dark-coloured.
"There are varying estimates of size, between five metres and 10 metres.
"That is consistent in description with a humpback and also other types, including a minke whale.
"We are working with British Divers Marine Life Rescue, who are monitoring the behaviour of the whale and the type of species.
"It is not an everyday sight, although we have had a number of whales in the Thames."
