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In photographs published by BILD, Sancho, Axel Witsel, Thorgan Hazard, Raphael Guerreiro, Dan-Axel Zagadou and Manuel Akanji were seen getting a trim from barber Winnie Nana Karkari at their homes without wearing masks.

According to DFL regulations, players should have no contact with the public, stay at home and receive no visitors.

In a statement released to BILD, Dortmund said: "It is pretty usual that our guys must get their hair cut after all these months. But it is surely a no-go, from both sides, not to wear a mask.

"We will remind our players about that very, very clearly. Also, the barber should ask himself about his behaviour too."

Karkari claimed he followed strict hygiene procedures and was wearing his mask at all times other than when pictured.

He said: "I only took off the mask for the photo, otherwise I followed all hygiene measures very precisely.

"I disinfected every machine before every customer, always cut with gloves even independently of coronavirus – and I am very, very fussy."

Dortmund fell seven points behind Bundesliga leader Bayern Munich after a 1-0 loss to Hansi Flick's side at Signal Iduna Park last week.

It is next in action at home to Hertha Berlin on Sunday (AEST).