Preston boss Phil Brown branded his side's 4-0 thumping at Millwall as "the worst performance" of his reign after their survival hopes were dealt a heavy blow.
Lions defender Darren Purse headed the hosts in front in the 42nd minute and Josh McQuoid added a second two minutes later. After the break John Marquis added a third and James Henry's penalty sealed the rout to leave Preston five points from safety with only three games remaining.
Brown said: "If we play like that again then we will be going down. But you won't be getting that repeat performance I can assure you."
He added: "I can't answer why we got that display.
"That is the worst performance I have seen in my time in charge and it was completely unacceptable."
Millwall boss Kenny Jackett praised rookie goalscorers McQuoid and Marquis after the Lions moved to within two points of sixth-placed Nottingham Forest.
Jackett said: "We have a lack of strikers for one reason or another at the moment, but the two young lads, Josh McQuoid and John Marquis, came in and did brilliantly.
"They don't have many league games between them so to look that threatening shows the players they are.
"They looked a natural partnership out there and what more can you ask, they both got goals and looked dangerous throughout."