Manager Michael Appleton admits Portsmouth are looking at League One football for the first time since 1983 after a 1-0 home npower Championship defeat to relegation rivals Millwall.
Pompey could have moved to within seven points of the south London club with victory at Fratton Park, but instead fell nine points adrift of safety with only four games remaining.
"I'm realistic enough to know that we're going to have to win four games and unless we do we're going to be looking at playing in a different division," said Appleton, whose side were beaten by Harry Kane's first-half strike.
Andy Keogh controlled a long ball eight minutes before the interval, twisted into space and teed up the Tottenham loanee, who stepped inside his marker and rifled a finish into the bottom-left corner from 20 yards.
In an otherwise uninspiring affair, Pompey had plenty of territory and possession but lacked a cutting edge. After a brave 2-2 draw at rivals Southampton on Saturday, this looked a game too far for many of the Football League's smallest squad.
"It's obviously disappointing," added Appleton. "We started the game really well, created a few good opportunities, got in behind them, but the first goal in football is so important and we've allowed the centre half to lump a ball up the field and given a really poor goal away.
"You could see in the second half the confidence oozing away from the players and it wasn't nice to see given the amount of effort they've put in over the last few weeks."
Millwall assistant boss Joe Gallen was full of praise for on-loan goalscorer Kane, after a tireless performance from the 18-year-old and his strike partner Andy Keogh.
"I don't want to talk him up too much because he's decent," said Gallen. "He's a very good finisher, very good all-round game and we're thankful to Tottenham for letting us have him.
"I think he's going to score lots of goals in his future. He loves football, he loves training - he gets annoyed when we pull him away from finishing training because we're worried he's going to get a thigh strain."