Manchester United fans travelling to their Europa League match against Feyenoord on Thursday to have been advised to dress "conservatively."
The letter from the Premier League club comes in the wake of safety concerns in Rotterdam following crowd trouble involving the Dutch side.
The Times reported that United supporters have been granted 1,400 tickets for the game at the De Kuip stadium, which is set to host just half of the usual 52,000 capacity.
The Dutch club are to reduce their capacity in a bid to avoid triggering suspended punishments they were handed by UEFA for crowd trouble.
The letter, reported in the Manchester Evening News, reads: "For your own safety it is advisable to maintain a low profile and avoid areas of potential conflict and by dressing conservatively.
"Once the game has ended, and Manchester United supporters are all out of the stadium, a special 1,200 seat train service will be provided from the stadium rail station. The service will stop at both Rotterdam and Amsterdam train stations.
"Please ensure that you carry your passport at all times, as required by law in the Netherlands. There is currently a general high threat level from terrorism in Europe and you should ensure that you are mindful of your own personal safety and security whilst travelling.
"Finally, we hope that you enjoy your visit to the Netherlands and thank you for your support."