Pompey lost 3-2 at home to Peterborough, while the Black Cats were beaten 2-1 at Fleetwood Town.
Luton will be back in the second tier following a 12-year absence, while Barnsley have secured an immediate return after relegation last season.
The Hatters are now favourites to seal the League One title on Saturday.
Barnsley, level on 91 points, could still overhaul them even if Luton are victorious at home to Oxford United, but only with a heavy win at Bristol Rovers.
The Tykes are currently four goals worse off on goal difference and have scored less too (79 to the Hatters’ 87), so they would have to win by a margin five goals better than whatever Luton manage.
If Luton fail to win, Barnsley would simply have to better their result to finish as champions.
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