UPDATED 16.10pm: Fears are growing for the health of a large whale trapped in the Thames in Central London.

A rescue attempt of the 17ft (5m) mammal, a northern bottle-nosed whale usually found deep in the ocean is about to start.

The whale has passed the Houses of Parliament and is moving upstream. Today it was stranded at least twice in shallow water, each time struggling free.

"This only increases our concerns," said Tony Woodley of the British Divers Marine Life Rescue (BDMLR), which is leading the rescue.

"This animal is causing us great concern as it is totally out of its natural environment, being the deep water of the North Atlantic."

A rescue boat is following the whale to protect it from crowds of onlookers. It has come within yards of the riverbank and bumped into an empty boat, which caused it to bleed lightly from the tail.

The rescue crew will try and re-direct the whale downstream with specialist equipment, including inflatable tubes.

A whale rescue pontoon system, which can be used to re-float the animal if it strands, is being brought to the scene.

"We won't be attempting to handle the animal while its still free swimming," Mr Woodley added. That would cause it "great stress".

"It is also quite dangerous to try and handle an animal of this size in the water".

Vets on standby

Expert vet Paul Jepson is on standby at the scene, with more trained volunteers on the way. If the whale gets stranded, they will assess its condition.

Should the animal be in good health, the team plan to transport it in the pontoon to deep water far out in the Thames Estuary.

But if it is ill, it would have to be put down to "prevent it from further suffering", Mr Woodley said.

Reports of a second whale near Southend-on-Sea are also being checked.

First sightings

At around 10am this morning expert Liz Sandeman of the charity Marine Connection approached the whale on a lifeboat with the Royal National Lifeboat Institution. At that time it still appeared to be healthy.

The first sightings of the whale were recorded yesterday, when the BDMLR received several calls about up to three of the mammals spotted near the Thames Gateway barrier.

At 8.30am today a member of the public phoned to say one of the mammals was swimming near the London Eye.

As the water quality in the Thames improved in recent decades, the river again became home to an array of species, including dolphins and seals.

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