Barcelona realistically required victory in their Group C encounter with Inter at Camp Nou to remain in with a shot of avoiding an early elimination for the second season running.
However, Nicola Barella and Lautaro Martinez struck after Ousmane Dembele had given Barca a half-time lead, before Lewandowski levelled up with a deflected shot.
Xavi’s men went all out for a winner and were caught out by Robin Gosens late on, though Lewandowski’s second goal did at least keep them in the competition for now.
Inter are three points better off than Barcelona and can eliminate the Catalan giants from the competition with a home win against pointless Viktoria Plzen in two weeks’ time.
Lewandowski took his tally to 14 goals in all competitions this term – only Erling Haaland has scored more in Europe’s top five leagues – but it was a day of disappointment.
“We wanted to score so much that we forgot about the defence,” the Poland international told Movistar+. “Inter got through on goal too easily and often had a man over in attack.
“We’ve had a lot of injuries in recent weeks and it’s not easy to change the line-up all the time. We’ve lacked stability and that maybe explains the situation.
“Inter are tough opponents for any striker. They defend with five players and also have a defensive midfielder.
“We tried to attack with many players but it meant we lost coverage. We scored three goals but conceded the same amount. We were disappointed not to get the win.”
The high-scoring draw with Inter was only the fourth time in their Champions League history Barca have conceded three goals in the second half of a game.
It also marked the first time in six visits to Camp Nou in the competition Inter have avoided defeat, paving the way for the Nerazzurri to wrap up qualification at Barca’s expense.
Gosens, the scorer of the visitors’ third goal, told Inter TV: “This was a match between two sides that wanted to win. We showed we have the right spirit to do well at Camp Nou.
“This is one of the most important stadiums in the world and not everybody comes away from here with a point. We stayed in the game and played like a real Inter side.
“This result must now stimulate the team to give even more. We know we’ve hurt a team like Barca and this must encourage us to do even more.”