The Blues lost 3-2 against Peterborough at Fratton Park at the end of last month, a defeat which means they will miss out on finishing in the top two spots in the league.
Portsmouth can still seal promotion to the Championship but they will have to make it successfully through the play-offs.
Here’s what you need to know:
How do the League One play-offs work?
The teams that finish first and second in the league win automatic promotion to the Championship for the 2019-2020 season.
Pompey’s defeat last night as well as Sunderland’s loss at Fleetwood, meant that Luton and Barnsley have sealed their places in the top two.
The teams below them will have to compete for the remaining promotion spot.
Which teams are in the play-offs?
The teams that finish 3rd to 6th will take part in the play-offs, competing for the third and final promotion spot for the
The team in third will play the team who finished sixth, while fourth place will play fifth place in the semi-finals.
Where are the semi-finals played?
Both semi-finals will take place over two legs, with teams playing both home and away.
The winners of the semis will advance to a final on neutral ground.
When are the League One semi-finals?
Saturday, May 11 – 7.30pm – League One A 1st Leg
Sunday, May 12 – 12,.30pm – League One B 1st Leg
Thursday, May 16 – 7.45pm – League One A 2nd Leg
Friday, May 17 – 7.45pm – League One B 2nd Leg
Sunday, May 26 – 3pm – League One final
The A and B games will be decided by the EFL after meeting with the clubs involved and the police.
Portsmouth vs Sunderland is the A game.
Where is the League One final played?
It will take place at Wembley, same as the League Two and Championship play-off finals.
Will the games be on TV?
Yes Sky will be broadcasting all of the EFL play-offs, so all four semi-final matches and the League One final will all be shown live on TV.
Do away goals count in play-off semi-finals?
Nope there is no away goal rule in the EFL play-offs.
What happens if it is all square on aggregate?
If the semi-finals finish level after the two legs, then it will go to 30 minutes of extra-time and if the teams are still drawing then it will go to penalties.