The Polish international took to social media to tell supporters that he will "give everything to BVB (Dortmund)".

"I want to address you directly. I have six months left here during which we all have the objective of achieving together," wrote the 25-year-old who is the joint top-scorer in the Bundesliga this season with 11 goals.

"Even if some of you do not agree with my decision, I hope to have your support. I will give everything for BVB."

Lewandowski, who was back in training with Dortmund on Sunday watched by around 200 fans, had a deal with the club only until June.

"We have known for a long time that Robert will be playing for another club in the summer. The confirmation of the transfer changes nothing," said Dortmund coach Jurgen Klopp.