David James saves at the feet of Christopher Samba
Blackburn all but sealed their Premier League place with a comfortable win over a Pompey side still desperately seeking the win they need to survive.
Morten Gamst Pedersen put Rovers ahead after David James had flapped under pressure from Christopher Samba.
Benni McCarthy made victory safe from the penalty spot after Hermann Hreidarsson had needlessly handled.
John Utaka wasted a chance to set up a tense finish when he hopelessly blazed over from the penalty spot.
Rovers' win means they can only now be relegated on goal difference, though a Monday night draw between Newcastle and Middlesbrough would guarantee safety for Sam Allardyce's side.
And it was a victory they undoubtedly deserved after outplaying the visitors for the majority of this match. After a spiky start which saw McCarthy and Glen Johnson booked inside the opening seven minutes for late tackles on a pitch made treacherous by constant rain, the game suddenly came to life midway through the first half.
McCarthy had threatened to get on the end of a hanging El Hadji Diouf cross and Diouf himself had flicked an effort across the face of goal by the time Pedersen made the breakthrough on 31 minutes.
Steven Warnock's cross was chested down by Samba and the giant makeshift striker's ominous forward lunge clearly struck fear into goalkeeper David James.
The England keeper, whose rick had gifted Arsenal their opening goal in a 3-0 win a week earlier, made a timid attempt to block Samba and could only parry the ball into the path of Pedersen, who gratefully poked the ball past Sylvain Distin on the line.
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It was the Norwegian's first goal since the final day of last season - having averaged nine a season for the previous four years - and a bizarre celebration bore the hallmark of a player who had forgotten what it is like to score.
Portsmouth, who knew victory would guarantee a seventh successive season in the top flight, responded as Hreidarsson saw a header from Sean Davis's free-kick flash just wide.
But Rovers looked more likely to add to the score and Warnock was desperately unlucky to see a volley on the stretch cannon off Kaboul's backside and behind for a corner.
Samba had a header ruled out for a push on Sol Campbell but Rovers' dominance was rewarded with the second goal they craved just before the hour mark.
McCarthy had his back to goal just inside the Pompey penalty area and looked to play a ball out wide to Diouf, only for Hreidarsson to lean across and block the ball with an arm.
Benni McCarthy celebrates after sealing victory from the spot
It was inches inside the box, but a clear penalty, and McCarthy clinically claimed his 10th Premier League goal of the campaign as he sent James the wrong way from the spot.
Portsmouth boss Paul Hart, with goal difference a growing concern as well as the failure to find the one remaining win they need for survival, made positive changes from the bench, introducing Jermaine Pennant for Kaboul and moving Johnson to right back.
With Kanu and Utaka having already provided a double Nigerian substitution, the new additions sparked something of a revival from the south coast side.
Peter Crouch set up Pennant for a cross-shot which looked certain to be turned in at the far post by Hreidarsson, only for the defender to make contact with the woodwork, but not the ball.
Hart admits Pompey still in danger
Then Kanu sent clear his fellow sub Utaka, who rounded Robinson only to see a wayward shot scrambled to safety by the back-tracking Rovers captain Ryan Nelsen.
Pompey's hapless afternoon was complete when Utaka wasted the chance to set up a grandstand finish after Andrews handled Kanu's cross.
The striker's spot kick was high, wide and hopeless and leaves Portsmouth nervously looking over their shoulders for the final two weeks of the campaign.
Blackburn manager Sam Allardyce: "This has not been a great escape, it has been survival we worked hard for over several games.
"We have been clear for a while and are nearly there with two games to go. Forty points guarantees nothing but I don't think we can be caught now."
Portsmouth manager Paul Hart: "You have to be on your toes against teams like Blackburn. They are hard to play against but I felt we gifted them two goals and that is disappointing.
"I have said all along that there will be no complacency from us in our preparation. If anyone thought we were safe already then today is a timely reminder that is not the case."
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