When the veterinary team got to the whale for a second time it had beached itself on East Winner Sandbank and was lying on its side in a rut it had created thrashing about trying to free itself.

Once the injection had been given it took about 20 minutes for the mammal to die. Richard Edward, a veterinary surgeon at Alphapet Veterinary Clinic in Bognor Regis, gave the injection.

He said: "It's a very potent injection so when I give it we need someone there ready with the antidote just in case I accidently inject myself.

"It immediately sedated the whale so it felt nothing and gave it a peaceful death. Without us stepping in it could have taken about a week to die.'

Once everyone was sure the whale had died it was attached to a buoy until RNLI crews could get to it and tow it back to land. Last night it was taken to a landfill site where the post-mortem will be carried out.

Paul Jepson, a vet from the Zoological Society of London, said: "We will be looking to see if it was ill, or another reason that caused the whale to swim here."

Samples will be sent for testing with results expected back in a few weeks.