Fears had been growing for the 16-18ft (5m) northern bottle-nosed whale which came as far upstream as Chelsea.

But now rescuers searching the east London stretch of river hope the sighting was accurate and it will continue to swim downstream.

Experts feared the animal could put itself in danger by becoming beached.

Tony Woodley, of the British Divers Marine Life Rescue Group, said it was spotted by a member of public at around 2045 GMT in Greenwich.

"Our rescue boat is now in the area looking for it with our spotlights on and if we see it, we will check its health."

The whale was spotted swimming past the Houses of Parliament

Teams had been becoming increasingly worried as there had been no sighting of the whale since 1815 GMT at Chelsea Bridge.

This latest development will give hope to the hundreds of thousands of people who have been following the progress of the whale, which is usually found in deep sea water.

It was first spotted at 0830 GMT on Friday by a man on a train and has since attracted massive public and media attention.

Crowds gathered along the riverbanks to witness the extraordinary spectacle.

But it soon became clear there was cause for concern, as the animal came within yards of the banks, almost beaching, and crashed into an empty boat causing slight bleeding.

It is a very long way from home and we don't know why it has ended up here

Alison Shaw, London Zoo

Whale in pictures

He said if it appears thin, it may be coming in to die, and added: "In which case we will euthanize it and consider it a success because it has not caused more suffering."

If well however, a system of pontoons will be used to try to refloat it by lifting it into a boat and taking it out to sea, he said.

"It is a very unusual thing and we have never done it before with a whale this size. It might float, it might not."

He added that the whale was damaging itself by coming ashore.

The RNLI say it is the first whale rescue on the Thames. A spokesman said three whales were spotted east of the Thames Barrier on Thursday but only one managed to get upstream.

Alison Shaw, of the Marine and Freshwater Conservation Programme at London Zoo, said the northern bottle-nosed whale was usually found in groups of three to 10.

She told the BBC News website: "This is extremely rare in British waters as they are normally found in deep waters in the North Atlantic.

"It is about 16-18ft long, so is relatively mature. It is a very long way from home and we don't know why it has ended up here."

The whales usually weigh about seven tonnes, which will complicate any rescue attempt, experts said.

Over the years dolphins and seals have been spotted in the Thames.

Sperm whales have been seen in the Thames Estuary and porpoises have feasted on fish near Vauxhall Bridge, in central London.

Hyperoodon ampullatus Adult length: 7-10m (23-33ft) Weight: 5.8-7.5 tonnes Diet: Squid, fish Habitat: Deep offshore waters Range: Arctic and North Atlantic Status: Lower risk, conservation dependent, protected since 1977 Distinctive feature: Bulbous forehead

Do you have any pictures or moving footage of the whale? If so, you can send them to yourpics@bbc.co.uk or MMS them to 07725 100 100