Danish midfielder Christian Eriksen was the hero as Chelsea slumped to a first defeat to Brentford since 1939.
Eriksen inspired the Bees to a 4-1 win that should seal their Premier League survival.
The European champions had gone in front early in the second half at Stamford Bridge through Antonio Rudiger’s sensational long-range strike, but they collapsed to lose for the first time since owner Roman Abramovich was hit by UK government sanctions.
Vitaly Janelt leveled just a minute later to spark a three-goal spree in 10 minutes for the visitors.
Eriksen has been in fine form since making his return to competitive football after suffering a cardiac arrest on the field during Euro 2020.
The former Tottenham midfielder scored twice in as many games during his international comeback for Denmark and continued his scoring streak by lifting the ball over Edouard Mendy to complete the turnaround.
Janelt then made it 3-1 with a deft dink over Mendy on the hour mark.
Third-placed Chelsea remains on course to finish in the top four but will need a major improvement for the visit of Real Madrid in the Champions League quarter-finals on Tuesday.
Yoann Wissa rounded off a memorable afternoon for Thomas Frank’s men when he took advantage of more comical Chelsea defending to drill home a fourth.
A third win in four games takes Brentford up to 14th and 11 points clear of the relegation zone.