Ilkay Gundogan
Borussia Dortmund midfielder Ilkay Gundogan certain to leave the Bundesliga sideGetty

Borussia Dortmund are adamant Ilkay Gundogan will be sold this summer, insisting the prospect of the Manchester United target leaving the Westfalenstadion is "unquestionable".

Reports in the German press in April strongly suggested that United have sounded out the possibility of signing the midfielder this summer, having been reassured of his long-term fitness following a return from injury in the 2014-15 season.

Gundogan added further weight to those claims when he refused to sign a contract extension at the Bundesliga giants, with just a year to run on his current deal.

Dortmund's sporting director Michael Zorc earlier this month admitted Gundogan's departure was likely but added "it will depend on offers that reach the club".

CEO Hans-Joachim Watzke has been firmer in his desire to find a new club for the Germany international, however, adding a number of clubs are now interested in the player.

"It is unquestionable that he will leave Borussia Dortmund this summer," Watzke told Bild. "It was not our wish to see him leave but it was his wish. Then we said, 'okay, then we will sell you and focus our plans differently'.

"I am quite confident that there are multiple clubs who want him."

Arsenal have also been credited with keen interest in the midfielder, while reports in Spain have suggested the player has already undergone a medical with Barcelona ahead of a move in 2016 when the Catalans are once again able to sign players.

Dortmund have already admitted they expect to lose two players this summer but are confident of avoiding a complete overhaul of the squad.

While club captain Mats Hummels also remains closely linked with a move to Old Trafford, his partner in defence Neven Subotic will not be going anywhere, having penned a two year contract extension.

Like Gundogan, the 26-year-old's current deal will expire at the end next season, but the Serbia international has committed to a club he describes as his "second home".