Manchester United have moved to avert potential trouble by warning supporters of the “real danger” of being attacked before their Champions League quarter-final, first leg against AS Roma in the Italian capital tomorrow evening.

After the incidents of crowd violence in Italy in February, when a policeman was killed, not to mention the history of mayhem between English clubs and Roma in Europe, United have taken the extraordinary step of issuing detailed letters to travelling fans, warning them about the threat of hooliganism, crime and the “primitive conditions” of facilities in the Olympic Stadium.

United are taking about 4,500 supporters to Rome and a further 1,500 are expected to travel without tickets for a match that could generate a hostile atmosphere in a city famed for its culture and elegance.

Fans have been urged to take the public transport shuttles to the ground and not to use the metro trains to the Piazzo Faminio, or the Pontenenni bridge, because these are the most common routes taken by Roma’s notorious “Ultras”. The Campo de Fiori has also been singled out as a no-go area by United.

Last season, three Middlesbrough fans were stabbed and ten were injured in the square during an ambush by about 80 Ultras the night before their Uefa Cup tie. Liverpool have had problems dating back to the 1984 European Cup final, when numerous stabbings were reported. Fourteen Liverpool fans were stabbed by Roma fans before and during a Uefa Cup tie in 2001.

While the incidents that blighted United’s 1-0 win away to Lille in the previous round, when supporters were fired on with teargas by riot police at the Félix-Bollaert Stadium in Lens, are fresh in the memory, the death of Filippo Raciti on February 2 after Catania fans went on the rampage at a derby match against Palermo and Roma’s supporters’ poor record of behaviour has put United on alert. Supporters will be kept in the stadium for 90 minutes after the match has finished.

The letter read: “Fans of other English clubs who have made their way independently to the stadium have reported on a number of occasions problems with getting back to the city centre after the game and there is a real danger of being attacked by the ‘Ultra’ fans of AS Roma. We realise that fans travelling independently may not wish to give up that independence, but we ask you to consider your own safety and welfare as being of paramount importance.”

Greater Manchester Police are sending three officers to Rome, although there is a fear that the death of Raciti has resulted in a hardening of the Italian police’s zero-tolerance approach.

Last night, Roma fans inundated websites with ominous messages about planned acts of violence, although Rosella Sensi, the Italian club’s managing director, said she did not expect any trouble. “I don’t perceive any tension,” she said.

On the pitch, United will not be helped by the absence of Nemanja Vidic, who was confirmed to have suffered a broken collarbone during the 4-1 victory against Blackburn Rovers at Old Trafford on Saturday. United hope that the Serbia defender will be back in “four or five” weeks, although it is unlikely that he will play again this season.

Sir Bobby Charlton has hit out at Real Madrid and Barcelona for attempting to unsettle Cristiano Ronaldo.

“I think anybody associated with Manchester United would be perturbed when, a few months before the end of a very important season, someone starts to make claims for Ronaldo,” Charlton said. “He’s our player and while he’s under contract to us I don’t think it’s anybody’s right to come and hopefully start the process of trying to change him into being one of their players.”