The Port of London Authority recovered the whale using patrol boats and a crane
A humpback whale found dead and floating in the River Thames is thought to have starved to death.
Scientists said the whale was spotted on Thursday and then not seen again until it appeared near Dartford on the Kent/London border on Saturday.
Toxicology tests are being carried out, but no food was found in its stomach and it has been assumed it starved.
Experts from the Zoological Society of London (ZSL) said it could have got lost on its first exploration alone.
Scientists from the ZSL and members of the British Divers Marine Life Rescue first received reports on Thursday that a whale had been spotted.
After it was found dead on Saturday, the Port of London Authority recovered the 28ft (9.5m) young male using a patrol boat and crane.
A ZSL spokeswoman said: "It's really unusual in this area. It could have been lost.
"Juvenile males go off on their own. It could have been its first lone exploration into the ocean."
Tests would not be able to reveal whether it had been disorientated, but samples had been taken to the laboratory for toxicology tests, she added.
According to ZSL, this was the first time a humpback whale had been found in the Thames.
The last humpback whale found stranded around the UK coast line was in 2007 at Port Talbot in Wales.
Scientists said it was "incredibly unusual" for a humpback to appear in the area.
Humpbacks are normally found through the world's oceans, using summer feeding grounds in Polar waters, and winter feeding grounds near the Tropics.
The ZSL, which manages the UK Cetacean Strandings Investigation Programme (CSIP), added that over the past 20 years, there had only been 12 strandings of humpbacks.
Manager of the programme, Rob Deaville, said: "Although it's obviously a sad outcome in this instance, the post-mortem examination has given us a rare opportunity to examine a truly extraordinary animal at close quarters.
"Information gathered through examinations like these will hopefully help further our understanding of such animals and also help contribute to improving their conservation status."