The northern bottlenose, which became lost in Langstone Harbour, is expected to be given a lethal injection after the tide has fallen and scientists can reach it easily. The whale was refloated after initially getting stuck on mud flats earlier but it was too weak to head out to deep waters.
It is likely it became dehydrated when it was stranded at low tide, resulting in muscle damage, which in turn could have led to the renal problem, experts told reporters.
Another theory is that the whale was near death when it entered the harbour.
"He is really out of its area here," a spokeswoman for the RSPCA told Sky News.
"He is now swimming about in shallow waters, but now we've got the test results back it is telling us that if he strands, which is highly likely, at low tide this evening that really the only best way forward for him is to put him down."
The animal is the same species that was found in the River Thames in 2006.
Despite a massive rescue attempt to save its life the whale died.
(Reporting by Avril Ormsby; Editing by Steve Addison)