Matt Smith headed in from close range to complete his hat-trick and take his season tally to 11 goals

Matt Smith scored a hat-trick as Millwall boosted their Championship play-off hopes and delivered a blow to Nottingham Forest's promotion challenge with a convincing win at the City Ground.

Forest had hoped to move to within five points of the top two with victory over the Lions, but instead found themselves 3-0 down inside 33 minutes.

Smith put the visitors in front in the 20th minute with a brave diving header before deflecting in Jayson Molumby's shot to double the lead.

And the powerful striker completed a 13-minute hat-trick when he climbed above Joe Worrall to head in Shane Ferguson's corner from close range.

Victory lifts Gary Rowett's side up to seventh in the table and within two points of the play-off places, while Forest remain fourth, eight points adrift of second-placed Leeds.

Worryingly for the Reds, they have now only won once in six games since beating the Whites at the City Ground last month at a time when they would have hoped to keep the pressure on their rivals.

Forest began brightly, with the recalled Joao Carvalho, making only his ninth Championship start of the season, shooting straight at Lions keeper Bartosz Bialkowski early on.

But Millwall, who had only scored three goals in their previous seven games, always carried a threat on the counter attack and were clinical in the way they took their first two goals, with Mason Bennett at the heart of both.

The on-loan Derby forward drove forward and fed the overlapping Murray Wallace in the build-up for the first, with the full-back then cutting inside Ben Watson before crossing for Smith to head in from eight yards.

Then Bennett himself exploited space down the Forest right and pulled the ball back for Molumby, whose sweet first-time side-footed finish from 15 yards took a nick of Smith's heel on its way past Forest keeper Brice Samba.

Sammy Ameobi saw a shot from 25 yards deflected narrowly wide shortly before half-time as Forest sought a way back into the game.

And although the home side dominated possession and territory after the break, the closest they came was when half-time substitute John Bostock twice had long-range shots pushed away by Bialkowski.

Forest appealed for a penalty when Carvalho appeared to be hauled down by Shaun Hutchinson as he shaped to shoot but referee Geoff Eltringham dismissed their claims and Millwall held on in relative comfort.

The Lions are now unbeaten in their past five league games against Forest and have lost only once in previous 10 meetings.

Forest head coach Sabri Lamouchi told BBC Radio Nottingham:

"It's a bad feeling and disappointing because the expectation was different. We gave them the possibility to kill us and they killed us. Congratulations to Millwall. Physically it was a big impact from them and and we suffered a lot.

"Mentally when you are physically down you need to play more simple. It was the wrong performance and result. It's a bad moment. We need to refresh and come back in training and find the solution for the next game.

"We have done a good job until now and we must stay together. We were not dangerous. We need to play with more passion. Millwall were better physically, the mentality was more aggressive and they had better determination.

"After eight months maybe some players are a little tired, so we need to think about that."

Millwall manager Gary Rowett told BBC Radio London:

"We were superb in different ways today. In the first 45 minutes, from an attacking sense, I thought all three goals were very, very well worked and we had some other opportunities which was pleasing.

"We could have come here and gone 5-2-3, but I felt there was an opportunity against Forest to exploit certain areas and one of those was a little bit of direct play on the angle to big Smudge (Matt Smith).

"I have asked the strikers to score goals and he has stood up. I have to work the team around Smudge because he is a big threat at the moment. Sometimes it means we have a little lack of energy in certain areas but he led the line absolutely magnificently.

"The lads worked really hard. The gameplan was spot on today. At 3-0 up we had to defend well under pressure and we did that. And we finished really well. They didn't create too many chances."