The humpback whale which died after swimming into the River Thames was hit by a ship, the Zoological Society of London has said.
However, it is not clear whether the wound on the juvenile female was inflicted before or after the whale's death.
The British Divers Marine Life Rescue (BDMLR), which has been dedicated to the rescue and well-being of marine animals in distress around the UK for more than 30 years, said it found the dead whale just after 5pm yesterday in the Greenhithe area.
Rob Deaville, ZSL's CSIP project manager, said: "ZSL experts from the UK Cetacean Strandings Investigation Programme (CSIP) are working with colleagues from Port of London Authority to carry out the post-mortem of the deceased humpback whale.
"From initial examinations ZSL can confirm the humpback whale is a juvenile female and has a large wound indicative of a ship strike, but it is currently unknown whether this was inflicted before or after the whale's death.
"ZSL's CSIP team will be carrying out the post-mortem this evening to learn more about the reasons for the whale's death and why it entered the Thames."
Humpback whales are rarely stranded around the UK coast, with just one or two recorded on average each year.
However, this is the fifth humpback to be recorded stranded in the UK so far this year.
Two other humpback whales which have previously been recorded in the Thames and wider Thames estuary region, in 2009 and 2013 - both died.
Sightings of the animal, nicknamed Hessy, were reported over the weekend and its presence in the river was confirmed by British Divers Marine Life Rescue (BDMLR) on Sunday.
But in an update on Tuesday evening, BDMLR said on Twitter: "Terribly sad news that soon after 5pm today the humpback whale which had not been seen in the Thames all day, was found dead around the Greenhithe area.
It was the first time in a decade that a humpback whale was spotted swimming in the River Thames.
While whales are sometimes seen off the British coast further north, national co-ordinator for BDMLR Julia Cable said it was "very unusual" for one to be seen within the Thames Estuary.
"It's very likely that it just made a navigational error," she added.
In 2006, a northern bottlenose whale was spotted in the Thames in central London.
On that occasion the whale, which was too weak to find its way back out of the river on its own, ultimately died as rescuers attempted to transport it back to sea.
And in 2009 a humpback whale was found washed up on the shore of the Thames in Kent, having seemingly died from starvation.
A year ago a beluga whale - which was given the nickname Benny - was spotted swimming in the Thames near Gravesend.
It is thought to have made its own way back out to sea in the new year.
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