|Ward 47, Arnason 85||Venue: New York Stadium
MILLWALL are now six points adrift of safety after suffering a catastrophic 2-1 defeat to relegation rivals Rotherham United.
The Lions had taken a lead into the half-time interval courtesy of Martyn Woolford's opener but the Millers turned the game on its head after the restart.
They equalised 90 seconds into the second period when the ball broke to Danny Ward in space and his drive deflected in off Jos Hooiveld.
Kari Arnason's towering header from a corner in the 85th minute completed the turnaround to leave Millwall in deep trouble.
With a trip to promotion-chasing Middlesbrough coming up on Tuesday night, the Lions' hopes of beating the drop could fade further.
Millwall had injury doubts over Gary Taylor-Fletcher and Woolford heading into the match but both players were deemed fit enough to start.
Taylor-Fletcher had been nursing a swollen foot while Woolford bruised his ribs during the midweek loss to Sheffield Wednesday but the duo put themselves up to play.
And it was Taylor-Fletcher who had the first effort on goal of the afternoon with a looping header over the bar from Dan Harding's cross in the fourth minute.
At the other end, Ben Pringle, who turned down a January move to The Den, tested David Forde with a 20-yard free kick which the Millwall keeper palmed away at his near post.
The Lions took the lead shortly afterwards when Taylor-Fletcher headed Shaun Williams' cross into the path of Woolford, who controlled the ball with his chest before stabbing a volley into the far corner.
Forde survived a scare in the 28th minute when he was charged down by Matt Derbyshire, with his kick ricocheting off the striker and wide of the Millwall goal.
A couple of Rotherham half-chances came and went as Jack Hunt's cross narrowly evaded Danny Ward in the box and Derbyshire glanced a header wide from 10 yards.
The Millers were doing most of the attacking but the Lions were always looking to counter and they carved out a golden opening in the 36th minute which should have seen them double their lead.
But Gregory, having beaten two defenders to Taylor-Fletcher's flick on, was denied from point-blank range by Adam Collin.
The significance of that miss became clear just two minutes after the break when Ward's deflected effort flew past a wrong-footed Forde to level the scores.
Woolford almost restored Millwall's lead with a far-post header from Diego Fabbrini's cross but the winger's effort was hacked clear just in front of the line.
A lapse from Harding in the 59th minute presented Hunt with a clear path to goal but the right back's sweetly-struck drive hit the base of the post and Forde saved Ward's follow-up.
There were half-chances at both ends as sub Ricardo Fuller saw an effort deflected just wide and Ward, looking for his second of the afternoon, was thwarted by Alan Dunne's last-ditch block.
Neither side was particularly free-flowing in the final third so it was little surprise that the winning goal came from a set piece – and it was the hosts who grabbed it.
Pringle's corner was floated to the back post and Arnason climbed unchallenged to power the ball into the bottom corner, leaving the Lions deep in the mire.