Marine wildlife experts scrambled today to save the life of a rare North Atlantic whale that entered the Thames overnight and swam upstream past Westminster, leaving a second whale stranded off the Essex coast.

The unprecedented visit by the northern bottle-nosed whale - from an endangered species that breeds off the coast of Nova Scotia and is rarely seen in British waters - brought television crews and bystanders flocking to the banks of the Thames for an impromptu bout of whale-watching.

But Alan Knight, chairman of the British Divers Marine Life Rescue service, which looks after stranded cetaceans and immediately sent a team to London this morning, said that he feared that the whale would not survive the encounter.

The situation was further complicated by the appearance this afternoon of another, larger whale of the same species that was trying to beach itself off more than 30 miles away at Southend-on-Sea, at the mouth of the Thames.

Mr Knight said that the two whales might be able to communicate by underwater clicks, even at that distance, but was concerned that one's distress calls would attract the other and the whale at Southend could decide to swim up the river.

"We're in trouble," he told Times Online."We could find that the one in the Thames was making such horrendous noises that the other one will swim towards and then we would have two whales in the Thames."

The first sightings of the whales, last night at the Thames Barrier, was greeted with some scepticism. Although northern bottle-nosed whales are occasionally stranded off the British coast, the last one off southwest England in 2003, none has visited the Thames since records began in 1913. One did beach itself in Woolwich, South London, in 1899.

But a spokesman for the Port of London Authority said that there was a flood of sightings this morning after the whale swam upstream overnight.

It was initially identified by non-experts as the more common pilot whale, before an expert from the Natural History Museum identified it as a protected northern bottle-nosed whale. There were also sightings of at least two porpoises in the river this morning.

Mr Knight said that the whale swam more than 30 miles upstream from the estuary, passing Parliament and reaching the area around Lambeth Bridge. As the tide turned at around 11.30am, so did the whale, swimming against the tide, back towards the estuary.

As the tide rose, the whale turned and started swimming eastwards, against the tide. It tried repeatedly to beach itself this afternoon, only to be refloated by the rising tide. It suffered some superficial wounds during those beaching attempts and was bleeding from its underneath, although that did not worry the experts

Mr Knight said the whale was expected to beach itself properly at around 1830GMT, at high tide, after which it will be examined by Paul Jepson, a veterinary pathologist with the Zoological Society.

If it is found to be suffering and deemed unlikely to survive it will be humanely killed with an injection. If not, it will be refloated using inflatable pontoons and towed back out to sea, although the rescue team would have to wait until the following high tide to start moving it.

"At 6.30 tonight, somewhere on the Thames, we will have a beached whale," Mr Knight said.

But any rescue operation for the wounded mammal may be hampered by the darkness, the river mud and the Thames’ notorious undertow. “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.

The experts fear, however, that even if they can take the whale back out to the estuary mouth to join the other whale, it may not survive after that.

"We would want it to turn right and head down the Channel into the Atlantic, but that is full of ships. If it turns left, into the North Sea, that would be a nightmare, with sandbanks off Holland and Belgium and the east coast of England," Mr Knight said. "It should be in the North Atlantic, not in the River Thames."

The northern bottle-nosed is a deep-water feeder that can dive to some 3,000 metres for almost two hours - deeper and for longer than any other whale - and is rarely found in less than 800 metres of water. It is dolphin-like with a bottleneck-shaped beak and prominent bulbous forehead known as a melon.

Adult males can grow up to around 10 metres (30ft), although the one on the Thames appears to be smaller than that.

One of the first to see the whale as it reached Parliament was Charlotte Giddings, a 25-year-old civil servant who was walking across Westminster Bridge at 9.15 am when she saw something "rise up and disappear" in the waters below.

"At this point there were no boats and no other people watching so I thought it was probably a piece of wood, but then it rose up further out of the water blowing water out of its spout. It was so bizarre - all these commuters were marching past behind me and I was looking down at a whale," Ms Giddings told Times Online.

"I rang my mum and asked her to ring the river police and tell them there was a whale under the middle arch of Wesminster Bridge - a very strange request. A couple of other commuters stopped and looked over before the boats arrived and then larger crowds gathered.

"I am going whale-watching in Scotland in March and am pretty sure I won't see anything as close as that."

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