The UK's Foreign and Commonwealth Office is calling on Manchester United supporters to be vigilant when they visit Rome for the Champions League final against Barcelona on May 27.
As many as 30,000 British fans are expected in the Italian capital with 20,000 tickets already allocated to the club.
However, the FCO estimates up to a further 10,000 will travel without tickets.
Officials are concerned the sheer amount of fans heading to Rome could lead to safety issues.
The FCO also predicts the excitement generated by what critics are calling ''the dream final'' will lead to more last-minute decisions to attend, increasing the likelihood that British fans will experience difficulties.
Alastair Mackie, FCO sports liaison officer, said: ''This is a once in a lifetime trip for many fans and we want them all to have a fantastic time but it is important to make suitable preparations before travel.
''We have been working closely with the Italian authorities and the British Embassy in Rome to ensure that fans have support in Rome and access to all the information they need to ensure a safe trip.
''We would encourage fans to visit www.fco.gov.uk/football before their departure.''
The FCO are urging fans not to take the metro to the Piazzale Flaminio or to use the Ponte Duca D'Aosta bridge as they say there has been trouble along this route to the stadium on many occasions.
There will be an official meeting point at Villa Borghese and free transport to and from the Stadio Olimpico for official fans and between Villa Borghese and the stadium for independent fans.
Pierluigi Puglia, Rome Embassy press officer, said: ''We have worked closely with the Italian authorities in preparation for the event and we appreciate their excellent co-operation in developing the necessary safety measures for British fans.''
Mayor Gianni Alemanno said: ''Rome welcomes the British fans with open arms and we look forward to celebrating this great sporting festival together.''