An inflatable pontoon system was used to support the 15ft northern bottle-nosed whale, who was first spotted in the river on Friday, before a crane lifted it out of the water close to Battersea Bridge in central London.

The barge then started to move downriver towards the Thames Estuary where tests were to be carried out to determine whether it was safe to release it back into deeper water. However, if the whale is too weak, experts may decide to put it down.

Thousands of onlookers who gathered on bridges and the embankment cheered as the whale was moved using a sling which had been carefully wrapped around pontoons.

A sighting of the whale yesterday near Albert Bridge, in Chelsea, disappointed rescuers buoyed by an earlier report that it had been seen in Greenwich, which is closer to open water.

A whale has not been spotted in the river since records began in 1913.