The Bees pressed home their advantage late on at the Gtech Community Stadium, with three goals after the hour mark from Jaidon Anthony, Igor Thiago and Fabio Carvalho, o seal a first league home win over the Blues since 1938.
Defeat just down the road at Brentford means Chelsea head into the international break with just no win from their last three top-flight outings.
After winning their two opening games of the PL campaign, Chelsea's sole posiviiiity has come via an EFL Cup victory against Leeds United, as they progressed on to the last 16 of that competition.
The result also sets an unwanted record for Alonso and his players as they have now lost more Premier League matches in 2026 than they did in the entirety of 2025.
Two of those have come under Alonso this season and nine in 2025/26 with ether Liam Rosenior or interim had coach Callum McFarlane.