The group leading the rescue of the Thames whale have saved thousands of marine mammals since their organisation was set up in 1988 by a group of divers upset by a mass beaching of seals on the coast of East Anglia.

But the operation that the British Divers Marine Life Rescue (BMDLR) service will attempt tonight is sure to be, if not their most ambitious operation, then certainly their most public.

All day long, television news helicopters have hovered over the Thames beaming back live coverage of the plight of the rare northern bottle-nosed whale that was seen swimming past Westminster this morning and has spent the afternoon near Chelsea Bridge.

Tens of thousands of Londoners have taken time off for a spell of whale-watching, many simply abandoning their cars at the side of the road to take in the unprecedented sight of a whale in the Thames. For London traffic wardens, Christmas came especially early this year.

The whale, which was sighted with a larger whale of the same species last night at the Thames Barrier, has tried several times this afternoon to beach itself, lifted every time by the rising tide.

The BMDLR volunteers have avoided handling it so far, to reduce the amount of stress on the animal and also because it would be too dangerous to do so while it can still swim freely.

But when high tide comes at around 6.30pm the whale will be properly beached and Paul Jepson, a Zoological Society vet, will examine it for signs of distress or ill health.

If his assessment is that it cannot survive, then the whale will be quickly euthanased with a lethal injection. If not, the BDMLR team will try to refloat it, turning the five or six-tonne mammal over by hand in the shallow water and slipping one or two inflatable pontoons underneath it to lift it up.

The picture above shows a similar rescue operation on a smaller Minke whale at Loch Dunvegin in northern Scotland.

Then at the next high tide, shortly after sunrise tomorrow, they will tow it down the Thames into deep water in the river estuary, where the whale will enter a new stage in its battle for survival.

It is a risky operation with no guarantee of success. To reach its natural habitat in the North Atlantic, the whales - if they are reunited - have to head south and then turn westwards through the English Channel, one of the world's busiest shipping lanes.

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