The 28-year-old man, from Middlesex, who works in the security industry, was attacked at about 10pm in the Underground lavatory at Piccadilly Circus on 6 August.
Detective Sergeant Tony Gallagher, leading the investigation, said the man had gone to the lavatory when three men came in and threw him against the wall.
'One of them pulled out a four and a half inch (11.4cm) pocket knife and threatened him with it. At this point he believed that he was being mugged. He was told that they didn't want to steal anything.
'They dragged him into a cubicle and pushed his head down the toilet bowl. Two of the men held his arms behind his back, while the man with the knife pulled the victim's trousers down and raped him.'
Det Sgt Gallagher said the attack was not being linked to the previous five male rapes and one sex assault. The attackers were all described as black, about 5ft 9ins (1.55m) tall, and aged between 20 and 24. They were all wearing casual clothes.Reuse content