A spokesman for the charity group organising its rescue from the river in central London said the bottle-nosed whale went into convulsions as it was taken by barge into the Thames Estuary and died at around 7pm.

Chairman of the British Divers Marine Life Rescue group Alan Knight said: "We had a phone call from the vet saying that it had started convulsing and while he was on the phone it died.

"It was still about two hours away from the planned release place."

Another spokesman, Tony Woodley, said his hopes had been lifted that the animal could survive when he saw it being lifted on to the barge but he soon realised that it would be a struggle to keep it alive.

"It is such a shame and I know that so many people in the UK and around the world have been watching this," he told Sky News.

"It is such a great shame that on this occasion we have not been successful."