A track and trace worker was beaten unconscious after moving away from an unmasked man on a London bus.
The 63-year-old NHS employee was punched repeatedly to the floor then had his head stamped on five times during the unprovoked attack.
Police have released CCTV of the man they want to speak to following the clash on the Route 149 bus on 23 August at around 9.45pm.
The victim boarded the bus shortly before Seven Sisters station and stood by the middle doors of the bus, while the assaulter boarded the bus at the next stop, footage shows.
Around five minutes later, the younger man followed the victim to the back of the bus and without warning the man launched into a violent physical attack.
Investigating officer PC Bowman, from the Roads and Transport Policing Command, said: “Although there is no sound on the CCTV and all the victim remembers is waking up in hospital, we believe that this was all triggered by face masks.
“We think that the attacker, who had his face covering beneath his chin, took offence when the victim, who was fully covered, consciously moved away from him.
“When the attacker then follows the victim to the back of the bus, he can be seen pointing and gesturing at the victim’s face mask before he starts repeatedly punching and kicking him.
“This was a totally unwarranted violent assault and we urgently need to speak to this man.
“If anyone knows who he is or has any other information – or witnessed this offence – please call.”
The suspect is described as a black man aged approximately 23-25 years, tall and with an athletic/muscular build. He was wearing blue tracksuit bottoms, a white vest and white trainers.