A Tottenham Hotspur supporter has been seriously injured and nine others hurt in clashes with rival fans in Rome.
Spurs are due to play Lazio in the Italian capital later in a Europa League match, and hundreds of their fans are in the city.
The Spurs fan was stabbed at the Drunken Ship bar in the Campo de’ Fiori district and is understood to have suffered a serious injury to an artery.
Lazio’s “Ultra” hooligan supporters have associations with fascism.
Tottenham have traditionally drawn support from the Jewish community and some Spurs fans refer to themselves as the “Yid Army”. Peter Herbert, chairman of the Society of Black Lawyers, recently threatened to report them to the police.
Nine of the injured fans are believed to be British and the 10th is reportedly American.
The newspaper La Repubblica said “urban warfare” broke out at the bar, with up to 100 “Ultras” attacking Spurs fans.
It said Lazio fans were armed with knives, baseball bats, and knuckle dusters.
The BBC’s Rome correspondent, Alan Johnston, reported police as saying 30 people – armed with iron bars and with their faces covered – approached the bar where the English fans were drinking in the early hours of Thursday and fighting broke out.
Our correspondent said the bar was wrecked and five people were arrested.
All of them were Italians who the police believe to be Lazio fans.
A spokesman for the Foreign and Commonwealth Office said it was investigating and would provide consular assistance.
Spurs and Lazio played out a goalless draw in September in a game which was marred by racist chanting by some Lazio fans.
Tottenham defender Steven Caulker and his manager Andre Villas-Boas, speaking before the clashes, both voiced their hope that Thursday’s game would not be overshadowed by by monkey chants.
Caulker said: “I’m not worried about the atmosphere – I hope football does the talking. Italy is a great footballing country. Hopefully that will be the talking point.”
Villas-Boas agreed but said he felt the £32,500 fine imposed on Lazio for the chanting at White Hart Lane in September may not be a deterrent.