An England fan had his ear bitten off by another after the national side's World Cup game against Uruguay.
The man was left with half his left ear ripped off and bloodied in the incident, the Association of Chief Police Officers confirmed.
The incident happened before Uruguay and Liverpool striker Luis Suarez was banned from the competition for biting an Italian player but details of the attack have only just emerged.
Former Liverpool striker Stan Collymore tweeted that he had seen footage of the fan attack and claimed it would be "the biggest story in the UK, especially who did it".
Chief Superintendent Rachel Barber said: "We were made aware during the England vs Uruguay match in Sao Paulo that an incident had taken place which resulted in an England fan having a portion of his ear bitten off by another England fan.
"In the immediate aftermath, we managed to make contact with the victim and offered advice and support.
"The day after the attack, the victim chose to make a formal complaint to the local police, and they opened an investigation."
No arrests have been made yet but interviews with witnesses gave police a name for the alleged attacker, Ms Barber said.
Police went to England's final match against Costa Rica in Belo Horizonte in case the assailant travelled to the match but did not find him there.
A Foreign Office spokesman said: "We are aware of an incident in Sao Paulo on June 19 involving a British national and we provided consular assistance."
On the day of the incident England lost 2-1 to Uruguay at the Arena Corinthians in Sao Paulo, with two goals from Suarez.
The Uruguayan striker was handed a nine-game international ban and four-month suspension from all football after his third biting incident during a game.
TV footage showed him burying his teeth into Italian defender Giorgio Chiellini and leaving tooth marks on his shoulder.
He had previously been punished for biting PSV's Ottman Bakkal in November 2010 and sinking his teeth into Chelsea defender Branislav Ivanovic in April last year.
Video footage obtained by the Sunday People appeared to show the moment the incident took place on the top tier of the Arena Corinthians.
Several men can be seen in a melee before one is led away by stewards.
When asked what happened, the victim appears to tell the camera: "He bit off my ear and called me a black ****."
Blood can be seen dripping down his neck but he continues to smile and laugh.
The man who shot the video, who the Sunday People named as north London-based fan Billy Grant, can be heard remarking about the assailant: "He's been giving it a large, he's an idiot."
Six British police officers travelled to Brazil for the World Cup to provide a link between Brazilian police and authorities and the thousands of England fans.
A Home Office spokesman said: "A delegation of seven UK police officers and officials travelled to the World Cup, at the host country's request.
"They worked alongside the Brazilian police for each England match, providing a reassuring presence for England supporters and liaising between the Brazilian police and England fans.
"Such co-operation has worked well at overseas international matches and tournaments for a number of years as it did this time."