Vets were due to put down a sick whale tonight after it became stranded for a second time on a sandbank.
The northern bottlenose whale was originally stuck on harbour flats in Langstone, Hampshire, for around 12 hours.
Marine experts were about to put it down when it refloated in shallow water and headed into the Solent - one of the world's busiest shipping lanes.
Blood tests taken from the 26ft long mammal were sent for testing and showed it is suffering from irreversible renal failure.
It became stranded once again on East Winner sand bank, Hayling Island, this evening.
Faye Archell, director with the British Divers Marine Life Rescue organisation (BDMLR), said a team of vets were on their way to administer the fatal strength anaesthetic, called Immobilon.
"The decision has been made based on medical grounds," she said.
"It's the right decision for the whale. Unfortunately it's not the outcome people would want but it's the right decision for the animal.
"It's sick and distressed and it has come in for a second time. It has chosen to strand both times.
"We now know it's not a navigational error but it has stranded for a reason, it is sick."
The six-ton young adult will be dehydrated and hungry. Dangerous toxins will also have built up as its organs were compressed under its body weight during the first stranding.
The kidney failure was caused mainly because of dehydration because it had not eaten for at least 48 hours and the muscle damage from being stranded.
Around 50 people, including firefighters with mud rescue equipment, marine experts from BDMLR, Coastguard and RNLI staff were at the scene today along with a crowd of more than 100 bystanders lining the shore.
Ms Archell also helped with the rescue of a stranded northern bottlenose whale along the River Thames in London in January 2006.
The female died while being carried to safety on a rescue barge at Gravesend, Kent, and a post-mortem examination carried out at the Zoological Society of London showed the cause of death was probably dehydration.
The bones have joined the Natural History Museum's national research collection of 2,500 whale, dolphin and porpoise skeletons.
Vets Paul Jepson and Rob Deaville, from the Zoological Society of London, a BDMLR vet and team of marine medics were to carry out the mercy killing tonight.
The northern bottlenose whale, Hyperoodon ampullatus, is miles off course as they are usually found in deep water.
Their closest habitat is off the Bay of Biscay where they feed on deep water squid and can grow up to 30ft long.
Local resident Peter Holding was amongst those watching the rescue this morning.
He said the fire service turned up at 6.30am to start pumping water over the whale to stop the animal drying out in the morning sun.
He said: "The fire brigade turned up with several appliances. They went down with loads of hoses and connected them from the mill pond and started pumping water over the whale."
Rita Delahunty, of the nearby Ship Inn pub, said she believed it was the first time a whale had been seen in the waters around Langstone.
She said: "It's very, very unusual. We have had seals before but never a whale.
"You can see it from a distance stuck on the flats. I just hope they manage to get it back into the water."
Marie Stevens, RSPCA animal collection officer, said: "Luckily we don't get many things like this happening on the South Coast.
"A lot of people cannot quite believe that we have a whale down here.
"They are willing it to survive. There's a certain empathy with wild animals, particularly whales and dolphins. We have a special relationship with them."
But Ms Stevens she added: "The blood test results indicate that's why it was stranded in the first place.
"From what the vet said, it's going to die anyway. We are very disappointed."