Marine experts are preparing to examine the body of the lost whale that died in the Thames.

Efforts to take the 18ft (5m) northern bottle-nosed whale to deeper water in the Thames Estuary ended in its death at 7pm last night.

The endangered mammal took a sudden downturn while being transported downriver on a barge. Vets decided to put it down on "welfare grounds", but it suffered a fatal series of convulsions before they could do so.

Yesterday thousands watched on the river banks near Battersea Bridge as the animal was moved onto a special pontoon and then by crane onto the MV Crossness.

At first vet Paul Jepson was "cautiously optimistic" about the whale's condition. Yet it quickly weakened once it was out of the river.

Trained volunteers covered it with a blanket and continuously splashed it with water.

Initial plans were to transfer the whale to an "ocean-going vessel" and take it to deep waters off the south coast.

Yet as the whale deteriorated, experts decided to release it at Shivering Sands off the north Kent coast. Although the barge could only have reached shallower waters, it would have been a shorter journey.

"From the outset we always knew we were up against it and the odds were slim that we could successfully rescue this whale," said Dr Jepson.

"But we felt the best way to assess the animal condition was to get it on to the barge and start to implement removal from central London."

'It could not be grabbed'

On Friday night the whale struggled against the incoming tide. It was stranded briefly at least once in the Battersea area, where the water is 8ft (2.5m) at its deepest.

Rescuers said they could not "grab" it while it was free-swimming, as the three-ton animal could easily injure a diver or capsize a boat.

The rescue operation only started yesterday afternoon when it again became stranded near Battersea Bridge.

Crowds applauded as the animal was lifted onto an inflatable raft on the barge, which served as a makeshift mattress.

"We were very worried about its condition, as its respiratory rate was too high, but it was stable," said Dr Jepson. "Unfortunately it did deteriorate very quickly.

"A decision was made to euthanase the animal on welfare grounds. However, before this could take place the whale sadly died."

Post mortem to follow

Experts of the British Divers Marine Life Rescue (BDMLR), the charity that mounted the rescue, will now perform a full post mortem. The results are due within the next few weeks.

BDMLR chair Alan Knight said millions worldwide shared in the "helter skelter of emotions" over the past two days.

"It is sad that the whale died but we really did give it the best chance possible."

The whale was spotted for the first time by a man on a train at around 8.30am on Friday. It has since captured imaginations worldwide.