AC Milan slumped to a surprise 1-0 home loss to Parma on Sunday to leave city rivals Inter 10 points clear at the top of Serie A.
Parma defender Mariano Troilo's late header condemned second-placed Milan to a damaging loss in the title race, capping a week in which they also dropped points in a draw with Como.
Massimiliano Allegri's side had seen Inter stretch their lead with a 2-0 win over Lecce on Saturday, but could not respond as they suffered just a second league defeat of the season.
Milan, who lost Matteo Gabbia to a muscle problem in the warm-up, suffered another blow early on at the San Siro when Ruben Loftus-Cheek had to be stretchered off with a head injury.
The England midfielder was carried from the field wearing a neck brace after colliding with Parma goalkeeper Edoardo Corvi.
Italian media reported that Loftus-Cheek would need dental treatment and undergo tests in hospital for possible head trauma.
Milan almost found a breakthrough in the 36th minute when Alexis Saelemaekers picked out Christian Pulisic, but the American winger's shot was kept out by Corvi.
Pulisic fired a better chance narrowly wide on the stroke of half-time.
Milan pushed for a winner in the second period but struggled to create clear-cut chances.
Parma caught them cold in the 80th minute as Troilo headed in, although the goal was initially disallowed for a foul by Lautaro Valenti on goalkeeper Mike Maignan.
But after a lengthy VAR review, the referee overturned his decision to leave Milan's title hopes hanging by a thread.
