Valentin, 26, told Vice News that he was stopped by police and told that his face mask - which doctors told him to wear to protect his immune system - fell foul of the law, which carries an €80 (£70) fine.

However, police relented after he showed them his medication and looked up his blood test results on his smartphone.

Valentin, who has leukaemia, said: “The whole thing was quite a stressful experience, but in the end they believed me.”

He was warned by police that he would require a doctor’s certificate next time he wore the mask in public or he would receive the fine.

Police spokesperson Patrick Maierhofer said: “Covering your face for medical reasons is an exception. In such cases the police must be given credible proof that it is being worn for medical reasons.”