The Royals'Â hopes of retaining their crown were dashed in spectacular fashion.
After establishing a two-goal lead in the opening 23 minutes Mick Catlin's side conceded three quick-fire goals before half-time.
Though Jack Breed levelled the scores again with his second goal of the game two late goals sent the visitors crashing out.
Royals boss Catlin was left stunned by the manner of the defeat.
He really was shocked by what he saw.
Portchester must now regroup after some bad errors cost them.
Catlin said: '˜I am still in a state of shock..
'˜It was a crazy game of football.
'˜We went two goals up and were playing really well up to that point.
'˜Then some kamikaze defending allowed them to score three goals in quick succession.
'˜We made the errors and they punished us.
'˜Even then we still managed to get back to 3-3 and at that stage looked like the only side that would win it.
'˜But then Harry Birmingham got caught in possession and they ran through to score.
'˜It was a good end to end cup-tie for the neutral but not the right result for us.'
Breed gave the RoyalsÂ a 21st-minute lead which Mike Turvey extended two minutes later.
Hampshire Premier League leadersÂ PaulsgroveÂ enjoyed a thrilling 4-3 home derby win againstÂ Fleetlands.
Two James Hird goals, one from the penalty spot, put Paulsgrove 2-0 up.
Danny Lane extended their lead early in the second period before a frenetic final 15Â minutes saw the teams share four goals.
Adam Fisher scored for the hosts and the visitors replied through Jake Alford, Matt Andrew and an own goal.