Shaun Batt's late effort earned a win for Millwall as Watford missed a chance to narrow the gap to second place.
Batt tapped in from a Martyn Woolford header to leave Watford six points behind promotion rivals Hull - beaten at Wolves - with three games to go.
Lions goalkeeper David Forde denied Troy Deeney before Batt's winner.
Millwall, with games in hand, move four points above the relegation zone three days after their FA Cup semi-final defeat by Wigan was marred by violence.
A second successive defeat leaves Gianfranco Zola's Hornets with plenty to do to grab the remaining automatic promotion place after Cardiff secured promotion with a point at home to Charlton.
They dominated throughout at the New Den and Deeney drove over after just two minutes then had another shot saved by Forde's legs.
Nathaniel Chalobah's lobbed pass sent Mark Yeates clear but Forde pulled off another block.
For the hosts, James Henry headed wide from a Chris Taylor cross and after the interval Forde thwarted Matej Vydra with a save low down.
Forde then gave the visitors another opening when he charged out of his goal only to miss the ball but Deeney missed the open goal from a wide position.
Watford were punished when Manuel Almunia blocked Henry's shot and Woolford guided the ball into the path of Batt, who turned in from a couple of yards for his first Lions goal of the season.
Millwall assistant Joe Gallen:
"In the second half I thought the whole club seemed to galvanise.
"We did not have too many chances but the one clear-cut one we did get, Shaun put in.
"It was a huge relief, especially after the other results tonight. I haven't got a clue what the safety figure is, I've never seen anything like the division this year.
"But a win against Huddersfield on Saturday would take us away from them and up to 58 points, and then we might be all right."
Watford boss Gianfranco Zola:
"We had two clear penalties not given. One when a header was stopped with a hand, and one on Troy Deeney in the first half.
"And it's not the first time it's happened. It looks like they are enjoying themselves when they do not give us anything.
"It's not the first time and we are missing points that would have made a big difference. It's game after game.
""It looks like the play-offs for us, although the maths don't say that yet."