Barcelona are in danger of failing to make it out of the Champions League group phase after a 1-0 loss to Inter Milan on Tuesday left them with just three points from three matches.
Hakan Calhanoglu scored the only goal of the game at the San Siro in first-half stoppage time to deliver a potentially all-important result in Group C.
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Bayern Munich, who beat Barcelona 2-0 last time out, top the group on a perfect nine points, with Inter second on six, ahead of Barca. Viktoria Plzen are still without a point.
Inter and Barcelona will meet again next week, this time at the Camp Nou.
Barcelona have now failed to score in four of their last five Champions League games. And despite Xavi Hernandez's side having almost 70% possession throughout Tuesday's encounter, it was Inter who had the clearer opportunities on goal in the first half.
Calhanoglu delivered a warning shot in the seventh minute when his first-time blast from 30 yards required a diving save from Marc-Andre ter Stegen to tip it over the bar.
Right before half-time, the Turkish midfielder gave Ter Stegen no chance from just outside the box. Barcelona were unable to clear the ball before it was eventually worked to Calhanoglu who drilled a shot into the bottom corner.
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Inter had the ball in the net even earlier, too, only for the offside flag to go up and rule out Angel Correa's effort after he broke clear and rounded Ter Stegen. Barcelona also survived when a penalty awarded to Inter was called back after an offside in the build-up was spotted by VAR.
It took until the 61st minute for the visitors to seriously threaten the Inter goal. A neat Barcelona move saw the ball shifted to Ousmane Dembele on the left of the box but his powerful effort came back off the base of the near post.
As Barcelona continued to push forward, Dembele was prominently involved again minutes later.
The winger's run and cross deflected into the path of Pedri who rolled the ball into the empty net. However, a VAR review showed that the ball struck Ansu Fati's hand before reaching Pedri, meaning Inter retained their narrow advantage.
VAR again frustrated Barcelona in stoppage time at the end of the game.
Play was stopped as officials reviewed a potential handball in the penalty area from Inter's Denzel Dumfries to redirect a cross but no penalty was awarded, leaving Barcelona facing an uphill task in their final three group games.