Tottenham loanee Harry Kane's 20-yard strike in the first half was enough for Millwall against a toothless Pompey outfit, who are now nine points from safety with only four games to go.
The Fratton Park side last played in the third tier of English football in 1982-83, but Millwall look unlikely to go down after pulling eight points clear of the drop zone.
Michael Appleton named an unchanged side from the one that valiantly secured a late point in the south coast derby with Southampton on Saturday, but this appeared to be a game too far for many of the Football League's smallest squad.
Millwall boss Kenny Jackett brought in Scott Barron for Hamer Bouazza, and his side defended efficiently, while forwards Andy Keogh and Kane were a constant threat on the break.
The first chance of the game came when Greg Halford played in Chris Maguire whose shot was well saved by Maik Taylor, and the 40-year-old was alert to keep out Luke Varney as the former Charlton man tried to prod home the rebound.
Halford tried his luck with a 40-yard volley in the 15th minute, although it was a straightforward save for ex-Southampton stopper Taylor and the former Wolves man headed a Scott Allan corner harmlessly over the crossbar midway through the first half before Maguire fired over from 25 yards as Pompey began to dictate proceedings.
At the other end defender Shane Lowry headed a corner over from inside the six yard box, before Pompey broke and James Henry almost headed a dangerous Joel Ward cross into his own net.
Scott Barron rifled a low effort which Jamie Ashdown was equal to just before the hosts' best opening of the game came on 34 minutes, as Karim Rekik played in Maguire on the left of the box, only for Taylor to get down well to stop his low effort.
Millwall went ahead eight minutes before half-time as Keogh twisted and turned on the edge of the box before passing to strike partner Kane, who stepped inside his marker and unleashed a fierce drive into the bottom left corner.
Wright almost doubled Millwall's advantage in first half stoppage time with a 30-yard volley that narrowly whistled over the bar.
Appleton, in the knowledge defeat would all but condemn Pompey to third tier football, brought on Hungarian striker Marko Futacs for defender Ricardo Rocha before the break, the Portuguese haven taken a knock moments earlier.
Straight after the break Varney thought he was clean through on Taylor, but the referee's assistant flagged for offside.
Five minutes into the second half Kane wasted a glorious chance to double the Lions' advantage. Having taken a long ball from Wright and beaten two defenders, he dallied over the finish and was eventually crowded out.
Futacs volleyed just over from 25 yards on 52 minutes, signalling the start of a sustained period of Pompey pressure.
However, the hosts' profligacy was summed up when after five minutes of good play, the ball broke to Allan 40 yards out and the West Brom loanee volleyed harmlessly over. Rekik and Varney then both shot tamely from distance as Pompey seemed to run out of ideas.
A Millwall break saw Kane head Wright's crossfield ball wide, before Portsmouth attacked again - Taylor gathering Futacs' powerful drive at the second attempt.
A scramble in the Millwall box saw the ball just fail to fall to Varney, and the home fans' frustrations were audible when Futacs slipped after being played through by an incisive David Norris pass.
Maguire thought he should have had a penalty late on after going down in the box, but referee Mark Halsey waved away the Scot's protests, while Varney's header from Halford's long throw deflected against the crossbar.
In the dying seconds Kelvin Etuhu wriggled in from the right and fired a 25-yard thunderbolt at the top corner, only for former Northern Ireland international Taylor to display his reactions with a fine save.