A post-mortem examination on the whale which became stranded in the River Thames showed it died from dehydration, muscle damage and failing kidneys.
The whale was female and aged 11 years
The preliminary findings showed it had been unable to feed for three days.
The 19.2ft (5.85m) female northern bottle-nosed whale died on Saturday as rescuers tried to take it back out to the open sea.
It first surfaced on Friday after mistakenly swimming into the North Sea and up the Thames, experts said.
They said it may have been trying to head west to the Atlantic Ocean where it could feed on deep sea squid, but made a wrong turning, ending up near Chelsea Bridge in the heart of London.
Without its normal squid diet, it was unable to rehydrate, they said.
Scientists examined the animal's body for up to eight hours at the dockside in Denton in Kent, and samples were then taken back to the Zoological Society of London (ZSL) for further analysis.
This showed it had a minor cut near her eye, but had been in good health and there was no evidence of any internal injury.
Further tests to be carried out over the next few weeks will determine whether there was any bacterial infection or pollutants in the whale's body, such as heavy metals.
In the meantime, the bones have been given to London's Natural History Museum and are in the process of being cleaned.
The entire skeleton will be kept for scientific research and made available to scientists from around the world.
On Saturday evening, the whale rapidly became disoriented and distressed.
It was then that it was decided to put it down for its own welfare, said ZSL's veterinary pathologist Dr Paul Jepson, who carried out the post-mortem tests.
"While I was drawing the lethal injection, she died," he said.
"The last few days have been an unforgettable and ultimately sad experience for us all, and we are now determined that the whale did not die in vain.
"The incident has demonstrated a clear message of the nation's passion for these animals and their conservation."
Dr Jepson dismissed speculation that the whale's death was caused by anti-submarine sonar from Navy vessels.
Following worldwide coverage of the story, the British Divers Marine Life Rescue group, the small charity behind the rescue operation has received more than £10,000 in donations.
The bill for attempting the rescue was about £5,000.