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Rescuers attach a flotation device to the whale on Saturday Jan. 21. (AP Photo/Tom Hevezi)

A northern bottlenose whale has died despite efforts to free the animal from the River Thames and take it to deeper water, one of the rescuers said on Saturday.

News of the death comes shortly after a rescue team hoisted the stranded whale onto a barge in central London, one day after it was spotted swimming in the river.

The young whale, normally found in the north Atlantic Ocean, was weak and distressed at the time of the rescue after having been in shallow water for so long.

Tony Woodley from the British Divers Marine Life Rescue (BDMLR), which was leading the rescue operation, had described the veterinarian on the barge as being "pessimistic" about the whale's chances.

The animal's condition had rapidly deteriorated during the transport and it was suffering muscle stiffening, the vet said.

The five-metre-long whale had attracted the attention of thousands of Londoners who had watched the rescue operation.

Crews first put the whale into a net, then refloated the animal with the help of pontoons and moved it downstream between two inflatable boats. Once it was moved, rescuers hoisted it up onto the barge in a sling attached to a crane.

Witnesses had reported seeing the whale swimming past Parliament and Big Ben, close to Albert Bridge on Friday.

It's not clear how the whale went astray. Witnesses reported seeing injuries to the mammal, claiming its snout was bloodied. Photos also appeared to show damage to one of the whale's eyes and a number of cuts to its torso.