Roque Santa Cruz scored his 21st of the season as Blackburn won at Fratton Park to virtually end Portsmouth's hopes of finishing fifth.
After a delightful one-two with Jason Roberts in the 73rd minute, Blackburn's Paraguayan striker claimed his sixth goal in the last seven games.
Santa Cruz left Jamie Ashdown - standing in for David James - helpless after the stand-in Portsmouth goalkeeper had seconds earlier punched away from the same player.
There was little meangingful action in the first half, but Jermain Defoe had thee efforts for Pompey and Ashdown had to save well from Roberts at the other end.
The second half was just as uneventful but Sulley Muntari scuffed a chance to put Pompey ahead just a minute before Santa Cruz's settled things.
Pompey must now beat Cardiff in the FA Cup final next month to get into Europe.