Leicester stunned Chelsea at Stamford Bridge as Jamie Vardy's second-half strike saw Maurizio Sarri's side defeated at home for the first time this season.
Vardy finished first time in the box from James Maddison's pass in the 51st minute after the Foxes had withstood heavy pressure.
Chelsea were dominant before the break and struck the bar through Eden Hazard, the best of a host of chances, that came following a Harry Maguire error.
But having gone behind it was Chelsea who became sloppy and Leicester only grew in strength.
Indeed, the Foxes might have doubled their lead but for Cesar Azpilicueta's sliding block to deny Vardy after Kepa Arrizabalaga had spilled a cross, before the keeper saved well from Marc Albrighton.
Chasing a goal of their own, Chelsea could not break down Leicester's resistance, despite creating two key late chances.
In the 89th minute, Antonio Rudiger nodded just wide from a corner, before Marcos Alonso struck the post with only the keeper to beat.
The Blues had not lost at home since 1 April, against Tottenham, and this defeat leaves them level on points with fifth-placed Arsenal, who beat Burnley 3-1 earlier on Saturday.
For Leicester, the win moves them up to ninth in the table and is their first victory from 11 matches in London.