The whale was found on a beach with "partial rigor mortis", said cetacean experts
The whale, among a 40-strong pod seen along the East Anglian coast, washed up near Goldhanger, Essex, on Thursday afternoon.
Marine experts successfully encouraged most of the whales from the shallow Blackwater Estuary into deeper waters.
But one - a 2.18m female - was found dead.
Rob Deaville, cetacean strandings programme manager at ZSL, said the whale was found on a beach with "partial rigor mortis".
"[This] indicates that the whale is likely to have died that morning, possibly around the time that the pod of whales was observed up the Blackwater River near Osea Island," he said.
"The whale had stranded alive before dying on the beach."
Tests showed the whale was in "very poor nutritional condition" with "no significant evidence of recent feeding".
He said: "The most likely cause of stranding and death at this stage is starvation, although we are waiting for the results from follow-up tests, including several to determine whether the animal had an underlying infection."
Rescue teams began their operation after the pod was spotted on Tuesday
Mr Deaville said: "In that time, only one long-finned pilot whale has been recorded stranded on the UK coast in the southern North Sea - a single individual in Norfolk in 1992 - indicating how unusual this event is."
The pod of whales has been off the Norfolk, Suffolk and Essex coastlines in recent weeks.
I feel really sad when I see all this whales and dolphins dying, I feel impotent. I feel shame.