Paul Cook admitted the players had let Pompey down after suffering defeat at relegation-bound York.
Jackie McNamara’s men inflicted a 3-1 loss on an abject Blues side on a night they could have exited the Football League.
During an appalling first-half from the visitors, Bradley Fewster and Lewis Alessandra netted in a four-minute spell.
Luke Summerfield added a third just after the break and despite Gareth Evans pulling one back the Blues were never going to restore parity.
The scoreline could have been even more comprehensive as Pompey continued to wilt under York’s attacks.
Now Cook’s men can forget all about automatic promotion as they seek to remain in the play-off positions.
The Blues’ manager said: ‘We have just come off the back of our most disappointing result of the season.
‘We had fantastic travelling support and they were an absolute credit to the club – unfortunately we have let everyone down.
‘As a manager you must stay as positive as you can be, I am not going to say what we have said and haven’t said.
‘York deserve great credit because we have been outplayed for long, long spells. We have conceded more chances tonight than I have seen us all season.’