Forest boss Sabri Lamouchi's side had no answer for the visitors' penetrative cut-and-thrust football in the opening half that allowed Smith to score the second hat-trick of his nine-season career, and his first in the league.

Forest had started brightly, with a Joao Carvalho snapshot from 16 yards straight at Bartosz Bialkowski in the fifth minute, while Joe Lolley rifled a drive from just inside the area over the crossbar.

Smith netted his first after 20 minutes, stooping brilliantly onto a low right-foot cross from Murray Wallace, who had cut inside Forest captain Ben Watson after Millwall had made a break from deep.

The next raid in the 26th minute was sparked by Jed Wallace dispossessing Alfa Semedo on the halfway line before playing in Mason Bennett on the overlap down the left-wing.

From Bennett's low cut back into the area, on-loan Brighton midfielder Jayson Molumby passed the ball into the back of the net from 15 yards, which would have been the 20-year-old's first senior goal in 34 appearances, only for Smith to claim via a decisive touch en route.

Just after the half-hour, a Millwall side that had only scored three goals in their previous seven league matches, made it 3-0, with Smith nodding home from two yards an inswinging corner to the far post from Shane Ferguson.

It completed Smith's first hat-trick since scoring all four for Bristol City in a 4-2 Johnstone's Paint Trophy win at Gillingham just over five years ago.

In need of a lifeline before the break, the closest Forest came was via a long-range effort from Sammy Ameobi that deflected off the back of Millwall captain Shane Hutchinson, and inches wide of Bialkowski's right-hand post.

It was not until midway through the second half that the home side conjured their next effort, with Bialkowski beating away another drive from distance, this time from John Bostock, who had replaced Semedo at the break.

After penalty appeals soon after for a Hutchinson shirt pull on Carvalho were dismissed, Bialkowski was forced into another fine save from a more stinging effort from Bostock in the 79th minute.

Smith was replaced to a standing ovation from the visiting fans 10 minutes from time of a game in which the Lions roared out a warning, leaving Forest limping along after claiming just six points from the last 18 on offer.