A wonder goal in the first half from Andres D'Alessandro gave Portsmouth a deserved lead and it looked as though they would get another victory in their amazing escape act, until two second-half goals in the space of seven minutes for the home side sent Harry Redknapp's side to what could prove to be a damaging defeat.
Eight points adrift of safety at the end of March Pompey, had picked up 14 points out of 18 going into the match, but even before the game started they lost defender Andy O'Brian to a hamstring injury in the warm-up.
Charlton boss Alan Curbishley, looking for his side to finish on a high note, had to make two changes in the first 40 minutes as Marcus Bent limped off with a hamstring injury and defender Jonathan Spector fell awkwardly and was carried off.
Former Addicks goalkeeper Dean Kiely was given a tremendous welcome on his return to The Valley and was a spectator in the first 45 minutes as he did not have a shot to save against an inept Charlton team.
The Portsmouth goal when it came was worth the wait, D'Alessandro curling in a 25-yard effort flying past the helpless Thomas Myhre.
Charlton went for broke after the break making their first substitution with Ambrose replacing Jerome Thomas and they at last begin to get their game together, with Kiely having to make his first save, tipping over a Gonzalo Sorondo header and deflecting Jay Bothroyd's shot for a corner.
Charlton equalised against the run of play when they were down to ten men with Hermann Hreidarsson off the pitch receiving treatment.
Darren Bent broke clear down the right and his low cross was tapped into the net from close range by Bryan Hughes .
Ambrose then had a shot deflected before the home side took the lead after some great play by Darren Bent.
The England international forced his way through the Pompey defence before firing past Kiely. He took his shirt off in celebration and was booked but that won't worry him as he celebrated his 21st goal of the season.
Portsmouth threw everything at the home side in the final period of the game and should have equalised when Azar Karadas put a close-range header past the post, with his team-mates looking on in despair.