A facebook post from Selsey Coastguard Rescue Team said the team were tasked with identifying and measuring the mammal which which washed up on Marine Beach. The team then sent the information The National History Museum.
It said: "Dead mammals can be full of dangerous toxins and other things, so please stay away and keep dogs on a lead to prevent them coming into contact with them.
"Dolphins, Porpoises and Whales are all protected under the Royal Fish laws of the United Kingdom and HM Coastguard undertakes duties on behalf of the receiver of a wreck to record details of Royal Fish washed ashore."
Anyone who comes across a Royal Fish is encouraged to contact 999 and ask for the Coastguard.