An England fan had part of his ear bitten off by a fellow supporter 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 striker Luis Suarez was banned from the competition for biting an Italian player. 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".
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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.