Pompey's winless year at home continued as Gillingham claimed their first-ever win at Fratton Park.
The Blues went down to a 3-1 loss as they threw away a first-half advantage and the Gills gained a first success at Fratton Park in 106 years of trying.
Jamal Lowe put Pompey in front in a bright opening 45 minutes in which they failed to make the most of the chances which came their way.
It was a different story after the interval, though, as Steve Lovell's men blew away the hapless home side.
Former Blues loanee Conor Wilkinson levelled three minutes after the restart with a fine effort.
The quality of that finish was then matched by two lovely long-range efforts from attacking midfielder Lee Martin.
There was a pre-match surprise as Jackett made four changes and handed Jack Whatmough a first start since September after his injury issues.
Sylvain Deslandes, Kal Naismith and Connor Ronan were the other players to come in as Jackett switched to a 3-5-2 formation.
The freshened Blues started powerfully, with Ben Close hitting a 30-yarder wide and Lowe heading a clear chance over from a few yards from Anton Walkes' cross.
Naismith was a whisker away from the opener with is 35-yard drive after 15 minutes - but the goal arrived four minutes later.
It owed much to Naismith's perseverance as he bundled the ball through to Lowe while on the floor. His strike partner side-stepped a challenged and blitzed home his finish.
Lowe should have bagged a second after 27 minutes, when Naismith put him clear one-on-one with keeper Tomas Holy.
The former Hampton & Richmond man dallied slightly, though, allowing Luke O'Neil to put in a last-ditch tackle.
Naismith had another couple of opportunities before the break as Pompey racked up the chances and were rightly clapped off on the half-time whistle.
Pompey's fine first-half work was undone three minutes after the restart as Gillingham levelled.
It was sub Wilkinson who got the goal, beating Matt Clarke and Christian Burgess and then angling a fine 15-yard finish beyond Luke McGee.
Connor Ronan nearly restored the home side's lead on 57 minutes when his quickly-taken free-kick hit the post.
But it got worse for Pompey after 66 minutes, as Martin angled a curling 25-yard effort which crept inside McGee's far post, as Wilkinson ran across the flight of the ball.
Dion Donohue had a decent chance with 16 minutes left when freed by Naismith.
The ball sat up nicely for the Welshman but his hit from 20 yards was the wrong side of the foot of the post.
Martin put the game out of sight for Pompey, though, with 10 minutes left with another gem as he curled a 25-yarder over and beyond McGee's dive to his left.
That made for a miserable finale again as swathe of Blues fans made their ways to the exit early - and boos on the full-time whistle.