It is the first time since 1913 that a northern bottlenose whale has been seen in the Thames, and hundreds of onlookers rushed to the riverside to watch as it progressed past parliament.

Experts say they are very concerned for the whale - normally found in deep seawater - and there was increasing concern about its chances of making it back to the sea, around 40 miles downstream.

There was speculation that if it became beached and stranded it could have to be put down in a mercy killing to minimise its suffering.

At one stage before 2pm, the 4.5 metre (15ft) long mammal came close to becoming beached near Battersea, coming within yards of the riverbank. A number of members of the public jumped into the river and splashed around to encourage the whale to move back into deeper waters.

It swam as far up river as Chelsea before lifeboats turned it around just short of Albert Bridge.

By 5pm, the whale - not yet thought to be fully grown - had moved away from the banks near Westminster and was spending more time beneath the surface.

Experts were at the scene, and a four-strong flotilla of boats - including a harbourmaster's vessel - kept watch on it.

Earlier today Liz Sandeman, a marine mammal medic who went out in a lifeboat to examine the whale, said: "It looks quite healthy and quite relaxed. It's breathing normally and its weight seems good."

However, a statement released by the British Divers Marine Life Rescue (BDMLR) later said the whale was showing increasing signs of "poor health".

Alan Knight, of the BDMLR, said there was currently a flood tide and the whale could become stranded when the tide changed.

Efforts could then be made to refloat it using specialist inflatable pontoon equipment that was being sent to the scene and could help direct it back towards the sea.

"We would only send it on its way if it has a good sense of survival," Mr Knight said. "If it does get stranded, it may have to be anaesthetised."

There is concern that boats in the river could pose a danger to the whale, with the noise of their engines adding to its disorientation.

It appeared to have received a small cut from rocks, and earlier today there were signs of blood in the water - but Mr Knight said this was not a serious concern.

He explained that reports of a whale making its way up the Thames had first been received yesterday.

"We had a report of two whales going up the Thames and we sent divers, but we only found one," he added. "At about 6pm [last night] it went back out again past the [Thames] barrier and we thought it was gone.

"Then at 8.30am today we got a phone call from someone on a train who thought they had just hallucinated and seen a whale going up the Thames near Waterloo Bridge."

Reports said a second whale, believed to be of the same species, had been seen at Southend-on-Sea.

Northern bottlenose whales are normally seen in the north Atlantic off Norway, in the Barents Sea and off northern Britain and Ireland in the summer.

They have bulbous foreheads, look like very large bottle-nosed dolphins and usually feed in deep water. The mammals can grow to 10 metres in length and are inquisitive, sometimes approaching boats in the north Atlantic.

A source from the Department for Environment, Food and Rural Affairs said: "Because this is a deep water creature coming into shallow waters, it is possible that its sonar has been affected and it could be disorientated."

Tom Howard-Vyne, the head of communications at the London Eye, said he had seen the whale on the opposite bank of the river to Westminster.

"I saw it blow, it was a spout of water which sparkled in the air. It was on this side of Westminster Bridge ... it was an amazing sight."