The six-metre-long Northern bottlenose whale had been lifted onto a barge by rescuers and was being taken downriver toward the North Sea when it suffered convulsions and died, the Royal Society for the Prevention of Cruelty to Animals said.
The whale struggled with the effects of being out of the water as it was ferried toward the Thames Estuary, officials said.
Swaddled in blankets on the rusting salvage barge, the marine mammal — watched by thousands in London as it spent two days swimming up the river past some of the capital’s most famous landmarks — had shown signs of increasing stress and stiffening muscles, an indicator it was in serious difficulty.
“We understand from the vet onboard the salvage vessel that the whale suffered a series of convulsions at around 7 p.m. (2 p.m. EST) and died,” RSPCA spokeswoman Katy Geary said.
Earlier, the whale was hoisted out of the river and onto the vessel to be sped toward the North Sea.
A crowd of 3,000 people at Albert Bridge in south London had cheered and applauded as the whale was tethered to a sling and lifted by a crane onto the barge Crossness.