Blackpool were relegated from the Championship after a 3-2 defeat against Millwall that saw the Lions climb into the play-off places on Friday.
The Seasiders needed a victory at Bloomfield Road to keep alive their faint hopes of avoiding the drop into League One.
But their two-season stay in the English second tier was ended by Zian Flemming's late penalty winner for Millwall, who moved into fifth place ahead of the rest of the weekend's matches.
Tom Bradshaw put Millwall ahead in the second minute with a clinical finish from Ryan Leonard's low cross.
Millwall wasted several chances to double their lead before Jerry Yates levelled for Blackpool with a 36th minute penalty after Jamie Shackleton's foul on CJ Hamilton.
FT | Blackpool 2 Millwall 3
— Blackpool FC (@BlackpoolFC) April 28, 2023
The Seasiders are relegated to League One. pic.twitter.com/8Y5hojGUL7
The south Londoners went back in front in the 59th minute as Bradshaw dinked a deft finish from Duncan Watmore's perfectly weighted through ball.
Blackpool hit back with a brilliant second equaliser after 66 minutes when Lewis Fiorini drilled a 25-yard drive home past George Long.
But Millwall condemned the hosts to relegation in the 75th minute when Watmore was tripped by Fiorini and Flemming stroked in the penalty.
Millwall moved up to fifth and are three points clear of seventh place, with a home match against play-off rivals Blackburn on the last day of the season.
Second bottom Blackpool are three points behind Huddersfield and two points behind Reading, but with the Terriers playing the Royals in the final game of the season it means the Seasiders are certain to go down.
Blackpool relegated to League One ???
— Sky Sports Football (@SkyFootball) April 28, 2023
Millwall get a crucial win to keep their play-off dreams alive ? pic.twitter.com/Hc55faDLI3