Police expect quarter of a million ticketless UK football fans at Euro 2016
Up to 500,000 British football fans, around half of whom do not have tickets, are expected to travel to France for Euro 2016, police have said.
Potential flashpoints include the England-Wales game in Lens, and England's fixture against Russia in Marseilles, which saw ugly clashes between home supporters and riot police in the 1998 World Cup.
British police are hoping to act as "cultural interpreters" to prevent heavy-handed tactics against drunk and rowdy fans who may not cause serious trouble.
Police delegations representing England, Wales and Northern Ireland are set to travel to the tournament, which has seen massive demand for tickets from the home nations.
French authorities have already told English and Welsh fans without tickets not to travel to Lens, and there will be an alcohol ban in the city centre for 24 hours from 6am on match day.
England and Wales were each allocated 9,000 tickets for the game, but there were more than 20,000 applications.
In total around 250,000 UK supporters have tickets, but it is expected that double that number of fans will travel to France.
Currently 1,841 England fans and 86 Wales supporters are under banning orders that apply from May 31 to July 11 and mean they must hand in their passports to police.
An operation covering 29 UK ports with officers from 19 police forces will launch on June 6, with specially trained "spotters" to detect hooligans trying to sneak out of the country.
Assistant Chief Constable Mark Roberts, the national lead for football policing, warned that the banning orders are no guarantee against trouble.
He said: "Some of the people who have drunk to excess and behaved in an anti-social manner are not known to the police. They are not people who we have on the periphery or the radar as saying these people will cause trouble at football. They don't have previous convictions sometimes in any matter, let alone football.
"We've got to be aware that even if you have a banning order operation that doesn't entirely guarantee that you won't have problems. And if we have up to 500,000 people travelling, it being summer they will no doubt drink, they will be in large groups, and we need to be aware that there may be potential trouble."
He also cautioned against allowing "keyboard warriors" to stir up trouble on social media ahead of the Russia game.
"Clearly we know there's a history there. I think we have moved a long way even since that tournament (1998 World Cup) in terms of the behaviour of English supporters abroad. It would be dangerous to make assumptions that it will be a repeat of anything that's gone before.
"It's important to be cautious about the keyboard warriors stoking this up in advance of it because these things can get a life of their own."
Plans are in place to fast-track banning orders for fans who fall foul of the law, to stop them returning to France for matches in the later stages.
Mr Roberts cited the example of Chelsea fans who were filmed racially abusing a commuter on the Paris Metro.
He said: "It's entirely conceivable that we will get an incident such as the one with the Chelsea fans, when they played Paris Saint Germain, on the tube, where something will be captured on social media.
"Obviously there will be an expectation for us to take action. This is France, we do not have jurisdiction in France, we cannot arrest people, we cannot use any sort of power on people.
"Our expectation would be if people are caught acting in that manner, of if there was some large-scale disorder, we would want to be in a position to take action in seeking a banning order and preferably as soon as this person came back."
The tournament is being carried out amid a severe terror threat, and French authorities have extended a state of emergency until the end of the event.
Mr Roberts said fans need to be aware that they will see military at transport hubs and paramilitary-style police.
"For the French it's going to be a massive security operation. It was described to us yesterday as a extraordinary scale of operation.
"We've got to accept that France has got its own policing style, it's a different country, so it's really for our supporters to be aware of that and to be respectful of the country that they're in and allow the French police to concentrate on keeping them safe.
"I don't think it's helpful to speculate on the policing style, I'm sure it will be as appropriate as it needs to be to the situation"
Concerns have been raised by French authorities that security is not tight enough at the Stade de France after fans were able to get pyrotechnics and glass bottles into the ground.
Mr Roberts added: "Any issues like that at a major event in France are always going to cause concerns. The consolation I think we can take is that this in effect was a test event for the systems they are going to have in place in the Euros.
"It is a concern but the solace is it's happened before the tournament and there is now the opportunity to take appropriate remedial actions."