At about 7pm (1900 GMT) the animal had convulsions and died.
Tony Woodley, spokesman of the British Divers Marine Life Rescue Group (BDMLR), said: "It was not put to sleep. It's dreadful news, but I'm afraid this is the kind of thing that can happen with these kinds of animals. And it's a dreadful shame."
He said he could not speculate on the cause of death. "It's probably multiple problems," he said.
Earlier in the rescue bid, spontaneous applause broke out as the 15ft (5m) adolescent northern bottle-nosed whale - the first whale since records began in 1913 to have been seen so far upstream - was laid on the padded deck to begin its journey to the sea.
As the barge sped towards open waters 70km downstream, volunteers poured water onto the whale to keep its skin damp and veterinary experts began assessing its health to see if it could be released or would have to be killed.
Time had been crucial in the rescue bid because a whale's body is not designed to bear its own weight and the animal can suffocate if kept out of the water too long.
The whale, one of the world's deepest diving mammals that usually travels in groups, triggered international interest when it was first spotted so far up the river on Friday morning.
Crowds applauded as the whale
was laid on the padded deck
Experts speculated that it was so far from its natural environment because it was either very ill, had got lost chasing food or had been driven from its usual habitat by military testing or loud sonar.
Whale expert Peter Evans of the Sea Watch Foundation praised the rescue operation that involved partially beaching the whale, moving it onto an inflatable pontoon and floating it down to the waiting barge.