Rescuers are planning how to help a whale that has been swimming in the Thames as far west as Chelsea.
Rescue teams are on standby if the whale strands
The northern bottle-nosed whale, not seen in the Thames for almost 100 years, has floundered in shallow water and looked to be bleeding at times.
Experts said heavy-lifting equipment might be needed if the whale beached when the tide turned.
However, they would give it a full health check before they decided whether to move it.
They have warned that it was likely to be ill as well as disorientated and may not survive the ordeal.
The Whale and Dolphin Society's Mark Simmonds said a rescue effort would be difficult.
"This is a very, very big whale to start manoeuvring around and lots of help and maybe even heavy lifting gear may be required.
"But they won't go to that point if they're not happy that the whale is healthy enough to be responded to in that way."
He warned that the animal may die.
"The prognosis is poor for this animal and the chances are that it is wounded, or distressed, or sick.
"So I am afraid I think people have got to prepare themselves that this animal may well not survive.''
The whale floundered in shallow water and was bleeding
A flotilla of four boats has been around the whale during Friday to protect it from other shipping.
Tony Woodley, from the British Divers Association, said he was concerned as the whale was a deep sea species not used to shallow water.
The association had volunteers trained to deal with whale strandings and a rescue boat at the ready.
Although he said the appearance of blood in the water might not be as serious as it looked, he also warned that the whale might not survive the ordeal.
The animal's delicate skin which is prone to abrasions, combined with "very red blood", made even a very small injury produce a large amount of blood, "and it could look possibly worse than it actually is" Mr Woodley said.
Two options available were refloating the whale back to deep water, which he described as very tricky, or putting it down.
"The other option is if the vet is of the opinion that the animal isn't going to survive...that we would actually put it down.''
The size of the animal would make any rescue difficult.
The curator of London Aquarium, Paul Hale, said: "This is a very active swimming animal and it's not going to go anywhere it doesn't want to go so we have to persuade it to swim back out.
"I think it's going to be a tricky time for the guys that are dealing with it."
The Zoological Society of London sent its marine mammal veterinary pathologist, Paul Jepson, to the banks of the Thames to assist with the rescue.