Sven Goran Eriksson has appealed for good behaviour from England fans at Euro 2004 in Portugal.
Sven has warned England fans
Eriksson said it would be a "real disaster" if England were thrown out because of hooliganism.
Meeting supporters' groups in London at a fans forum, the England coach said if fans behaved, they could act as the team's "12th man".
The forum is part of a wider approach to crack down on orchestrated football hooliganism ahead of the contest.
Banning orders to stop individuals leaving the UK during this summer's championship have been issued to 2,500 people.
About 50,000 England fans are expected to travel to the Algarve for Euro 2004.
But the police, the Football Association and the government have tried to ensure the hooligan ringleaders will not be there.
Eriksson told reporters: "If the thousands and thousands behave in the same way as the people here, it will be very good.
"It will be a small disaster if we leave early for
football reasons, it would be a real disaster if we do so for other reasons."
He told fans: "You are very important. You are our 12th player for us wherever we go."
BBC correspondent Nigel Adderley said Sven's meeting with fans was in effect "preaching to the converted".
But the England coach hoped it would help get the anti-violence message publicised.
The threat of Uefa ejecting England from the competition on grounds of violent behaviour is real, as England are on a final warning after previous violence.
It is serious, pre-planned hooliganism that is the major cause for concern.