Borussia Dortmund boss Hans-Joachim Watzke has reacted angrily -- but did not dismiss -- reports striker Robert Lewandowski is set to join arch-rivals Bayern Munich next season.

The 24-year-old Lewandowski is out of contract in June 2014 and has so far rejected Dortmund's offers to extend his deal, while reliable German magazine Der Spiegel have reported the Poland star will join the league leaders next season.

"All that is just pure speculation, there is absolutely zero substance in the primary source - that has really annoyed me and I will say nothing more on the subject," Dortmund's Chief Eexecutive Officer told SID, an AFP subsidiary.

"The fact is Robert has a contract until 2014 and we want to extend it."

The source of the Spiegel report is someone "with direct access to Bayern Munich's board room" and German daily Bild claim Munich would need to pay in the region of 30 million (US$40.6m) for the Polish international.

Lewandowski is one of the most sought-after strikers in Europe with both Manchester United and Manchester City also reported to be interested in a player who has scored 18 goals, with 11 assists, in 28 competitive matches this season.

Watzke has said Lewandowski "won't just settle for any old offer", but it would be an unpopular move with both Dortmund and Bayern fans alike, if the Bavarians sign their rival's top goal-scorer.

Munich fans protested openly when Germany's first-choice goalkeeper Manuel Neuer was signed from Schalke 04 in 2011 with a group of the club's hard-core fans even issuing the new arrival with a strict code of conduct to adhere to.

He was discouraged from celebrating with them, throwing his shirt into the crowd or even from kissing the club logo on his shirt.