Rescuers were tonight racing to save a whale swimming in the River Thames in central London.
Fears were growing for the 15 foot long northern bottle-nosed whale, as experts believe it may become stranded when the tide changes this evening.
The Northern Bottle-nosed Whale pokes its head out of the Eiver Thames near Battersea Bridge in south London
But any rescue operation for the wounded mammal may be hampered by the darkness, the river mud and the Thames's notorious undertow.
The whale has also injured itself by its attempts to come ashore and is bleeding.
It is the first time a whale has been spotted in the river since records began in 1913.
Alan Knight, of The British Divers Marine Life Rescue group, said: "The tide will turn about 6.15pm and will then start to flood out. If he is going to strand it will be around that time.
"If it actually goes ashore and strands we can get out of the boats and have a look at the animal."
Mr Knight said if the whale appeared thin "it might be coming in to die, in which case we will euthanize it and consider it a success because it has not caused any more suffering".
However, if the whale is assessed as being in the right condition, the team will attempt to refloat it using a system of pontoons, lift it on to a boat and take it out to sea.
"The problem we have got is actually trying to do that at night, in the mud, in the River Thames. There are certain problems in terms of safety," he said. He added, in the case of a rescue, the team would use a Port of London Authority launch with a crane to lift the animal - weighing up to four tonnes - aboard.
"It is a very unusual thing and we have never done it before with a whale this size. It might float, it might not.
"We are not going to put the animal through terrible stress. It is a delicate balancing act at the moment, and we are being advised by our vets," he said.