Pompey suffered their biggest defeat of the season as they lost 3-0 to Stevenage
On a day to forget for the Blues, they also saw defender Christian Burgess sent off for a second bookable offence on 75 minutes.
Meanwhile, substitute Conor Chaplin missed a penalty in the dying minutes, with the front man’s chipped effort landing in keeper Chris Day’s arms without him even moving from his spot on the goalline.
The win sees fourth-placed Boro close the gap on Pompey who remain third.
But Paul Cook’s side will be disappointed by their return from their trip to the Lamex Stadium, with the defeat opening the door for those teams directly below them in the League Two table.
Pompey were unlucky not to take the lead after five minutes when Gareth Evans’ shot from the edge of the area smacked against the crossbar.
But from that moment on it was Stevenage who imposed themselves on the game, with Matt Godden’s effort, which hit the side-netting, giving them the momentum.
The hosts were roused further when a goalmouth scramble led to Josh McQuoid’s effort being cleared off the line by Matt Clarke on 11 minutes.
And Boro got the goal their play deserved when Luke Wilkinson’s volley from tight against the touchline hit the crossbar and fell kindly to Jack King, who was on hand to apply the finishing touch.
The Blues’ response was an Evans effort that went widely over the bar.
But it was Stevenage who again posed the greater danger in front of goal and they added a second on 27 minutes through Godden.
Steve Schumacher’s delivery from a free-kick was only half-cleared by Noel Hunt.
The ball dropped perfectly to Jobi McAnuff, who found Godden with an inch-perfect cross to the back post, and the striker made no mistake to double the home side’s lead.
To Pompey’s credit, they responded well, with Carl Baker’s effort from a free-kick hitting the crossbar on 33 minutes.
However, a good finish to the half was quickly undone 41 seconds into the second half when McAnuff found the corner of the net after Forde had initially saved.
It could have got worse for the visitors when McQuoid’s rasping effort from outside the box took a deflection and dipped over the bar.
That prompted a double substitution from the Blues, with Nicke Kabamba and Gary Roberts replacing Hunt and Baker.
But there was no change in the formation, with a switch only occurring after Burgess was shown a red card following a second bookable offence.
From then on it was damage limitation for Pompey, and Forde had to get low down to deny Godden his second of the game.
The travelling Fratton faithful were given the chance to celebrate when Chaplin, who replaced Eoin Doyle on 73 minutes, was brought down in the box for a penalty on 84 minutes.
However, the youngster wasted his chance to reduce the arrears with an effort that will leave him deeply embarrassed.
Tying to be cute, the striker chipped his spot-kick goalwards.
It was easily read by Day, who remained rooted to his spot and collected the ball with ease with ease.
It summed up a bad day at the office for the Blues, who could have no complaints after Stevenage ran out deserved 3-0 winners.