A third defeat in four games, courtesy of goals from Kevin Schade and Yoane Wissa, leaves the stumbling two-time European champions outside the Premier League’s top five with just four games remaining.
Liverpool have already won the Premier League and second-placed Arsenal have a comfortable cushion but just five points separate third-placed Newcastle from seventh-paced Aston Villa.
This season, the top five will all qualify for the Champions League after strong performances in Europe from Premier League teams.
Not since 1980, under former manager Brian Clough, have Forest played in the continent’s top club competition. But after a season spent largely in the upper reaches of the Premier League they are suffering a major wobble at the worst possible moment.
“It’s a bad night and a bad game,” Forest manager Nuno Espirito Santo said. “I’m not really worried about the table, more how to correct the mistakes.
“Credit to Brentford but the situations we should solve normally with better defending. We are disappointed but it’s about reacting and going for the next one,” he added. “There are still four games to go, a lot of football to be played. We have to focus on ourselves and play better football.”
Forest would have climbed to third with a win but despite the defeat they have 60 points, just two behind Newcastle, with Manchester City, Chelsea and Villa also in the mix.
Brentford, themselves chasing a European spot after a fine season, went close to taking the lead in the 17th minute when Sepp van den Berg got his head to a free-kick from the right but he planted his header too close to goalkeeper Matz Sels.
Forest, who have made an art form of exploding on the break, worked their way back into the full-blooded contest.
Brentford made the breakthrough on the cusp of half-time when defender Nathan Collins fired a long ball down the middle to find Schade.
The Germany forward did not control the ball initially but Forest defender Ola Aina stumbled and Schade had a simple finish.
An initially balmy May evening turned to lashing rain at the start of the second half and Forest dominated the ball in the early stages, with Anthony Elanga going close.
Top-scorer Chris Wood headed Aina’s cross wide in the 65th minute as the pressure from Forest mounted.
But Brentford silenced the home crowd when Wissa doubled their lead with 20 minutes to go following another long ball.
Mark Flekken’s goal-kick was allowed to bounce and Wissa outpaced Nikola Milenkovic, before coolly chipping Sels.
Nuno made a flurry of substitutions but to little effect as Brentford closed out the game impressively.
Defeat is a bitter disappointment for Forest but a major boost for Brentford in their own drive to qualify for European football next season.
The three points lift the Bees just one point behind 10th-placed Bournemouth as they chase their highest Premier League finish.
“It is a big strength that we go to a difficult place against a team who have been fantastic all season,” Brentford boss Thomas Frank said. “I think everything is possible. There are four games left. We’ll try out best to win them.”
Forest’s remaining fixtures are against Crystal Palace, Leicester, West Ham and Chelsea.
Their season is threatening to end with a whimper after defeat in Sunday’s FA Cup semi-final to City. (AFP)