Police forces all over England and Wales have sent warning letters out to more than 200 people banned from attending this weekend's FA Cup semi finals at Wembley.
Special plain clothes officers will mingle with the crowd and try to identify fans on banning orders and potential troublemakers.
The Metropolitan Police are particularly concerned about intelligence they've received relating to one individual who may be travelling down from Manchester for the Man Utd-Man City clash.
He's described as being "5 feet 10 inches in height and 12 stones 5 lbs in weight. He is of stocky build and speaks in a Liverpool accent".
The Met have been advised that he is liable to commit a breach of the peace by either shouting and swearing into any TV cameras he passes or by provoking a fight with older fans by chatting up any grandmothers he encounters.
He may also try to buy his way out of trouble as he is liable to have in his possession a substantial wad of cash.
