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, after Napoli lashed out at the refereeing following defeat at Atalanta.
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 halftime.
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.
NAPOLI BLAST 'SHAMEFUL' REFEREEING
Napoli hit out at "shameful" refereeing after slipping to a 2-1 loss at top-four rivals Atalanta.
Raffaele Palladino's side extended their unbeaten run in the league to nine matches with an impressive second-half fightback.
Atalanta remain seventh in the table, but only two points behind fourth-placed Roma, who host Cremonese later on Sunday, in the race for next season's Champions League.
Injury-hit reigning champions Napoli sit third, but only five points above Atalanta and now 14 adrift of Inter.
Napoli were incensed after having a penalty award overturned and then a goal disallowed in the first half.
"Where is the foul? The VAR called him on the penalty, why not on this? It's embarrassing," Napoli sporting director Giovanni Manna told DAZN.
"There has to be a review of this because every week a club ends up here protesting about decisions.
"All we want is what is right, what is ours, but this is not right, and it is not football. It's shameful, I have no more words."
Atalanta will turn their attentions to next week's Champions League knockout play-off second leg against Borussia Dortmund, when they will attempt to overturn a 2-0 deficit in Bergamo.
The visitors grabbed an early lead when Sam Beukema, one of three Napoli players left completely unmarked, headed in Miguel Gutierrez's free-kick.
Conte's men were awarded a penalty shortly before halftime when Rasmus Hojlund went down in the box, but the decision was overturned following a VAR review.
Hojlund was in the thick of the action again in the first minute after the restart when his cut-back was turned in by Gutierrez, only for the goal to be ruled out for a foul by the Danish striker in the build-up.
Atalanta levelled just after the hour mark when Mario Pasalic flicked a header into the corner from Nicola Zalewski's corner.
The home team completed the turnaround with nine minutes remaining as Lazar Samardzic sent a looping header into the far corner from Lorenzo Bernasconi's cross.