Brett Williams’ stoppage-time header wrecked Pompey’s hopes of returning to the top of the League Two table tonight.
The substitute struck moments after coming on to the pitch to earn his Stevenage side a 1-1 draw and leave the Blues deflated.
That cancelled out Conor Chaplin’s fifth goal of the campaign after he entered the fray in the second half.
Chaplin’s 77th-minute strike saw Paul Cook’s men eventually make the breakthrough after assuming increasing control against Teddy Sheringham’s side.
But the late, late set-piece undid Pompey and meant Plymouth Argyle remain at the summit.
Cook made one change from the team who defeated Newport County with Rommy Boco making his first start in place of Kal Naismith.
Pompey had a good early chance when Gary Roberts’ angled drive was saved by Jesse Joronen after two minutes.
Tom Pett drilled a dangerous ball across goal after 17 minutes for the visitors before firing over the top from the edge of the box.
There were decent Pompey penalty appeals ignored after 23 minutes when Ben Davies went to ground under the attentions of Jerome Okimo but referee Andy Woolmer waved play on.
And Roberts should have done better in the 32nd minute when a lovely first-time touch from Bennett put him in after good work from Tubbs.
But he could only fire into the side-netting as Pompey finished the half the stronger.
It should have been the away side who were in front four minutes after the restart when a fine passing move put Dipo Akinyemi in but he dragged shot wide.
Tubbs then looked to have a tap-in from Roberts’ cutback but the ball got a touch off a Stevenage defender with the goal at his mercy.
Pompey upped the pressure as they went in search of the opener with Michael Doyle’s 25-yard piledriver bringing a smart stop out of Joronen.
And after a incessant spell on the front foot their efforts were rewarded through Chaplin.
It was Webster who slipped the ball in for the Worthing talent to beat Joronen at the near post after a slip from Jamie McCombe.
It looked for all the world as if it would be the match-winner until sub Williams levelled deep into stoppage-time.
A long throw from Charlie Lee wasn’t dealt with and Williams managed to lob a header over Aaron McCarey to flatten Pompey.