Oliver Bierhoff: Has rubbished claims of Bayern favouritism

Hummels told Kicker magazine that he had "had to work harder than others" to gain his place on Joachim Low's team, while also claiming it is impossible to speak your mind for fear of losing your place.

Bierhoff says that is not the case, however, and players are treated evenly, regardless of which club they play for.

"There's no such train of thought in the national team," he told Sport1.de. "We treat every player equally, it makes no difference which club they come from.

"Players from Dortmund, Schalke and Bayern all eat and play cards together at the same table."

Hummels is not the only person expressing concerns that Dortmund players are treated differently.

Ever since Low publicly criticised Marcel Schmelzer over a year ago, the Westphalians have been keeping a close eye on the Germany coach's words.

"Whenever there is somebody at fault, then it's not unlikely that it's a Dortmund player," said Dortmund coach Jurgen Klopp recently to Sky. "It surprised me a bit with Marcel, and now it surprises me a bit with Mats."

Dortmund's general manager Hans-Joachim Watzke has vowed to "extend our antenna" and monitor the treatment of their players on national team duty with Low set to name his squad for the final two World Cup qualifiers later this week.

One name which is almost certain to be missing is that of Dortmund goalkeeper Roman Weidenfeller, regarded by many as one of the strongest in the Bundesliga, but constantly overlooked by Low.

"There cannot be any sporting reasons for his exclusion so there's got to be something else, only I don't know what," said Dortmund's director of sport Michael Zorc to the Bild newspaper.