A man in a boater hat stormed into a pub toilet and punched two men in the face as they relieved themselves.
The victims, 40 and 38, had been using the gents' at the William Morris pub in Hammersmith when their assailant walked in.
Police said the attacker, who neither man knew, "got into a brief verbal altercation with the men who were using the urinals" before delivering the blows.
The incident left the older man in hospital with facial injuries.
Scotland Yard released a CCTV image showing a man they believe has information about the assault, which happened at about 10.50pm on July 7.
"The man is described as white, approximately 25 to 30 years and of medium build," said a police spokesman. "He was wearing a distinctive boater-style hat."
Anyone with information about the incident, or anyone who recognises the man in the picture, is asked to contact the Met's Hammersmith and Fulham team on 020 8246 2505, or phone Crimestoppers anonymously on 0800 555 111.
