LONDON, August 1, 2008 (AFP) - Vets gave a sick whale a lethal injection on Friday after a rescue operation to free it from mudflats in southern England ended in failure, rescuers said.

The six-tonne (5,440-kilogramme) northern bottlenose whale had been re-floated after it became stranded near Hayling Island in Hampshire for around 12 hours when the tide went out.

But it had already suffered irreversible kidney failure when it became stranded on another sandbank later in the day and vets noticed that water was starting to lap around its blow hole.

Faye Archell, the director of the British Divers Marine Life Rescue which led the operation, said the whale could not have survived even if it had returned to the open sea.

'A decision was taken to put the animal to sleep on welfare grounds. There's no coming back from renal failure. It's not good news but is what we expect from these cases,' she said.

Dehydration and muscle failure were the probable causes of the renal failure, Archell said.

The northern bottlenose is a deep-water whale which feeds on squid, so experts were unable to explain why it had swum into such shallow waters in Britain.

It was the same species as a whale that became stuck in the River Thames in London in 2006, sparking huge media interest in Britain. That animal died as it was being ferried back out to sea.