Security around Tottenham's Europa League game against Lazio on Thursday will be stepped up after travelling Spurs fans were attacked in a Rome bar.
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The Foreign and Commonwealth Office (FCO) said police had told them they were increasing security measures in the city.
Nine Tottenham Hotspur fans were injured after being attacked, with one in a critical condition after being stabbed.
An FCO spokesman said: "We are aware of a serious incident involving British nationals in Rome.
"We are in contact with the Italian authorities and the Italian police, who are investigating the incident and have confirmed there will be increased security measures for the game this evening.
"We have been in contact with one British national and have been updated by hospitals on the conditions of all the others, some of whom have already been released."
Up to 50 'Ultra' supporters of the Italian club carried out the attack, which happened in the Drunken Ship bar at around 1am local time.
The masked assailants stormed into the bar, in Rome's Campo de Fiori district, armed with baseball bats and broken bottles. They attacked the visiting fans and wrecked the bar.
Italian newspaper La Repubblica reported scenes of "urban warfare" and quoted one witness as saying: "We feared the worst."
Sources named four of the injured Spurs fans as Ashley Mills, Dave Lesley, Stephen Tierney and Christopher Allen. Mr Mills is reported to have been stabbed and is seriously ill in Rome's San Camillo Hospital.
A Tottenham spokesman said: "Three of these supporters were taken to hospital for further treatment. We continue to seek further information, and are liaising with both Italian and UK police on this matter."
Marco Manzi, the landlord of the Drunken Ship, said the Tottenham fans had been well-behaved and there had been no problems until the sudden ambush.
"The bar was full of about 30 English supporters at 1am. Most of them were aged between 40 and 50, and they were well behaved," he said.
"Then arrived what I presume were Italian supporters wearing scooter helmets with their faces covered with scarves carrying stones and rocks.
"They arrived in the pub and they attacked the English supporters. Some of the English supporters tried to escape down Via dei Cappellari and it was in this street where one of them was hit by a knife."
Police confirmed that a number of Italians had been detained in relation to the incident.
A spokesman said: "A group of about 40 or 50 people in motorcycle helmets or with their faces covered were armed and attacked the group of English football fans."
Some Spurs fans have voiced fears about security around the match. Colin McAteer, a 59-year-old fan who had travelled to Rome from Essex, said: "There are some coaches going up to the stadium, but what protection we'll have I don't know.
"We have heard stories that we are going to get picked off when we cross the bridge to the stadium. It's a bad time here. We are all completely shocked.''
The incident comes after Lazio were fined £32,500 by UEFA after some of their fans directed racist abuse at Tottenham's black players during the reverse fixture at White Hart Lane in September.
Information from the Press Association was used in this report