Manchester United have made a £21 million offer for Borussia Dortmund defender Mats Hummels, according to the German publication BILD.

The initial bid is expected to be rejected as the Bundesliga club are holding out for an offer of £32 million for the German international.

Hummels is under contract until 2017, but admitted last month he would consider a future away from Dortmund at the end of the season.

The prized defender has been strong linked with a move to Manchester United since last summer, with speculation of his departure from Dortmund increasing following the news of BVB coach Jurgen Klopp’s resignation last week.

Hummels won back-to-back Bundesliga titles in 2011 and 2012, reached the 2013 Champions League final and was named team captain last summer.

However, this year the team has struggled domestically and are likely to miss out on European football altogether next season.

If other reports prove correct, and United reach an agreement with Dortmund for Hummels, the defender would become the fourth summer signing for Louis van Gaal following the completion of the 2014-15 season.

Last month, Daily Mail reporter Mike Keegan tweeted that sources close to the club had informed him that United has already agreed to three deals for the summer.

While the names of the players were not released, speculation is the players involved are: Southampton right-back Nathaniel Clyne, PSV Eindhoven Memphis Depay and Athletic Bilbao center-back Aymeric Laporte.