Pompey’s draw with second-placed Barnsley means they were unable to close the gap on the top two.
Omar Bogle’s 61st-minute penalty miss ensures Kenny Jackett’s side still sit five points behind the Tykes in the League One table.
But while Daniel Stendel’s outfit managed to secure a point on their trip to the south coast, third-placed Sunderland closed the gap on them to two points after a 2-0 win at Bristol Rovers.
Luke O’Nien and Aiden McGeady scored either side of half-time for the Black Cats as they extended their unbeaten run in the league to 12 matches.
Jack Ross’ side have a game in hand on the Tykes.
Charlton, meanwhile, narrowed the gap on Pompey to three points with a 2-1 win at AFC Wimbledon.
Michael Folivi handed the hosts a 24th-minute lead, before Naby Sarr equalised six minutes after the break.
Igor Vetokele then netted the winner for Lee Bowyer’s fifth-placed side in the 90th minute.
The Blues head to The Valley on March 9.
Doncaster Rovers remain in the final play-off positions after they picked up a point from their visit to Scunthorpe.
Peterborough failed to overtake them in the table after they lost 2-1 at home to Shrewsbury.
League leaders Luton are at home to Coventry tomorrow (midday).