The Poland international has long been linked with a move away from the Bundesliga champions, with Bayern Munich and Manchester United thought to be at the front of the queue for his signature.
Zorc confirmed to Sky Deutschland on Wednesday that the player will not extend his current deal at Signal-Iduna-Park, and that he will likely leave at the end of the current season or after the 2013-14 season.
"Robert will not renew his contract, he will leave either this summer or in the summer of 2014," he said.
The news is likely to have caught the attention of officials at both Bayern and United, although Lewandowski is unlikely to be short of suitors following his form for Dortmund over recent seasons.
Germany legend Franz Beckenbauer claimed earlier this week that Jupp Heynckes's side does not need the 24-year-old due to the forward options already at its disposal, although he confessed he would be happy to see the player make the move.
Lewandowski joined Jurgen Klopp's side in the summer of 2010 in a 4.75 million euro move from Lech Poznan.