Selsey Coastguard said rescue teams were tasked to locate and assess reports of possible ordnance in the mud on Medmerry Beach last night (Thursday).
A spokesman added: "We managed to locate it and were able to confirm with our operations room that it was ordnance and measurements and photos were sent to the Explosive Ordnance Disposal Team (E.O.D).
"Speaking with E.O.D. and our Operations Room a decision was made to return first thing in the morning.
"The team were stood down and returned to station at 22.00."
Selsey Coastguard confirmed the team returned to the scene this morning and 'were able confirm that the ordnance was still there'.
"A phone call to the Navy E.O.D. Team was made and the beach was closed," the spokesman said.
"Once E.O.D. were on scene they assessed the situation and carried out a controlled explosion at approximately 8.30am this morning.
"The piece of ordnance found was highly explosive and we would like to remind everyone that ordnance comes in all shapes and sizes and this piece and previous ordnance found on this beach have been found to be highly explosive so please do not touch it and dial 999 and ask for the Coastguard straight away.
"In an emergency out at sea, on the beach, or along the coast dial 999 and ask for the Coastguard!"
It comes after two similar incident on Medmerry beach at the end of January: