The French government must ask itself "searching questions" after the "deeply disturbing" violence at Euro 2016, the Home Secretary said yesterday, as it emerged that authorities failed to arrest any of the Russian hooligans who clashed with England fans.
More than 150 Moscow Ultras, described by police as "hyper-violent", are feared to be plotting further attacks on Welsh and English fans in Lille after police were overwhelmed by scenes of violence inside and outside the Marseille stadium.
Yesterday six England supporters, including a psychiatric nurse and an apprentice engineer, were jailed for at least a month each and banned from France.
Theresa May said some England fans had behaved "inexcusably", but added: "The French and Uefa will rightly be asking themselves searching questions about how the segregation of fans within the Vélodrome stadium broke down."
Both the French government and Uefa are under pressure to avoid more trouble, with Russia's next game taking place just 24 miles away from England's match against Wales on Thursday.
Tonight the Welsh FA revealed French authorities were made aware of a "powerful group of Russian thugs" arriving in France last week.
Brice Robin, chief prosecutor of Marseille, admitted his officers were overwhelmed during four days of violence involving England supporters.
"I don't want to go as far as to suggest these are professionals but they are extreme and well-trained," Mr Robin told the media, as he reacted with shock to the ferocity of the Russian attacks.
The organised paramilitary-style Russian Ultras were "hyper-fast" and "hyper-violent", he added.
The squad that ambushed drunk fans during a wave of attacks inside and outside the Stade Velodrome all managed to evade police.
The worst injured England fan was named online on Monday as 51-year-old Portsmouth supporter Andrew Bache.
A well known Portsmouth fan affectionately nicknamed the "Pepe of Pompey", Mr Bache suffered a critical brain injury after he was hit several times by Russia supporters with an iron bar in the head.
Mr Bache was repeatedly hit over the head with what is believed to be a hammer or iron bar when he was set upon by the mob of Russians.
The Portsmouth Football Club fan was given CPR by French police - which saved his life.
Friend Jay Ricketts, 44, says Mr Bache was put in an induced coma by French doctors in a bid to reduce the swelling on his brain, following the "unprovoked attack."
Mr Ricketts has set up a fundraising page for Mr Bache, of Portsmouth, Hants, and nearly £6,000 has been raised so far.
Mr Ricketts said: "It's been difficult as none of his family are out there, it's difficult to get information.
"His brother and son are on their way now.
"As far as we know he's been put in an induced coma to help the swelling in his brain to go down.
"There's no real update on his condition as such time as they bring him out of that.
"His life was saved by the French police who gave him CPR.
"This was a completely unprovoked attack by the Russians. He sustained severe head injuries."
He added that Mr Bache - who is nicknamed Pepe - was in a group of about six friends who had gone out hoping to follow England's progress in the tournament.
He said: "He's a long-time Pompey and England fan, gone out to watch the match."
A second friend, who did not wish to be named, said the England fan was struck over the head up to five times.
He said: "He got caught up in it. He got smashed over the head with a hammer or iron bar four or five times.
"We're trying to get money up together to help with his mortgage. He was there with a group of friends.
"The Russians came into the square. It's quite sickening. Most people got away but he got caught up in it."
Mr Bache's brother, Dean, today told reporters he is flying out to France this evening to see his brother.
The 53-year-old, also from Portsmouth, said he did not wish to make any comments about his brothers injuries or the attack.
He added that once he has seen his brother, he will make a comment to the media.
The Shepherd's Crook pub in Portsmouth today revealed they will be holding a benefit night on Saturday to raise money for Mr Bache.
The landlord at the Milton Arms pub, where Mr Bache used to drink before Pompey games, said the family have asked them not to make any comments to the media.
Despite the behaviour of the Russian fans, who fought running battles in the streets and also charged at England fans in the Stade Velodrome after Saturday night's 1-1 opening draw, no Russians have been arrested over the violence.
Most of the Russians who caused trouble were known hooligans and authorities failed to take their passports away, the French police chief said.
He said about 150 Russian hooligans had been involved in the trouble, describing them as "hyper violent" and "hyper rapid".
They were among 12,000 Russians who were in Marseille over weekend.
But Mr Robin said just two Russian fans had been arrested, both for a pitch invasion.
The Press conference was held as it emerged a 16-year-old English supporter had been prosecuted for throwing a bottle at police and has had his ticket confiscated.
Meanwhile, a Northern Ireland football fan who died after a fall in France has been named as Darren Rodgers from Ballymena.
The victim, 25, toppled eight metres (26ft) over a barrier from a promenade onto a hard pebble beach in the south coast city of Nice at around 2am this morning, police said. It is understood the man was on his own at the time.
Officers issued pre-tournament advice to both English and Welsh fans - especially those without tickets - to stay in Lille, partly because match-day street-drinking is banned 24 miles away in the small city of Lens, where Wales and England meet in a sell-out encounter on Thursday.
However, in an apparent U-turn after four days of violence in Marseille, Mark Roberts, the lead for football policing in the UK, suggested fans should now consider also avoiding Lille as it hosts Russia's second game.
"It's realistic to expect that the Russian fans will seek to try to behave in a similar way," he told The Telegraph.
"The majority of Russian fans are no doubt decent folk, but there is a hardcore group of Russian fans who are willing to use extreme violence. They will be in Lille the night before and people should just bear that in mind."
Roy Hodgson, the England manager, and captain Wayne Rooney recorded direct pleas to fans not to get the team kicked out of Euro 2016.
Speaking after threats from Uefa in the wake of violence in Marseille, Hodgson said: "As a manager I'm obviously very concerned about the threat which is now hanging over us and the sanctions that could possibly be imposed upon the England team.
"We have worked very hard to get here and we really desperately want to stay in the competition.
"I'm appealing therefore to all of our fans, and we appreciate your support at the matches of course, but I'm appealing to you to stay out of trouble and try and make certain that these threats that are being issued are never carried out and we will be able to do the best we can to stay in this competition by football means."
Rooney said: "I would like to thank the England fans for the great support at the stadium against Russia.
"And now we have a big game coming up against Wales I would like to ask the fans please, if you don't have a ticket don't travel.
"And for the fans with tickets be safe, be sensible and continue with your great support for the players."
Meanwhile, England's footballers enjoyed a day off yesterday - with many of them opting for a game of golf near their five star hotel in Chantilly.
Many of the players headed for the exclusive Golf du Lys Chantilly course which is just a few miles from their Euro 2016 base.
There are two separate courses at Chantilly which includes the 18-hole 'The Oaks' which was designed in 1929 and a newer course opened in 1976.
The players arrived at the clubhouse at lunchtime and were all given buggies before setting off in to play two and three-ball matches.
England fans were advised before the tournament to stay in Lille ahead of the game against Wales in Lens's Stade Bollaert-Delelis, which is just 24 miles away.
The Home Office said it would be willing to provide extra officers to police future England matches amid fierce criticism of French authorities for failing to do more to protect innocent fans caught up in four days of violence in Marseille.
A Government spokesman said: "We have offered to send further UK police to France ahead of the next England game to support the security operation around the match in Lens.
"And UK police will be assisting the French with their post-incident investigations and supporting them to gather evidence, including evidence against any England fans involved in the disorder."
UK officers will automatically have a bigger delegation in Lille and Lens because they will combine spotters deployed for both England and Wales fans.
"If we receive a request for support from the French, we will provide whatever we can," ACC Roberts added.
Authorities in Lille appeared resigned to the prospect of further clashes between supporters.
Local security spokeswoman Veronique Planchon said: "We accept that there is a high risk that supporters of England and Russia will cross paths again in the week.
"There is no ban though on the sale of alcohol nor will there be the forced closure of any bars."
Lille has a much wider range of hotels and English, Welsh and Russian fans travelling from Paris will come out at the Gare de Lille Flandres station, which is a short walk from the city's Uefa Fan Zone in the Parc de Roubaix.
Locals expect most travelling fans arriving in Lille to congregate in the city's central drinking and eating area, which is based around Rue Solferino, a 20-minute walk through the city centre from the main train stations.
Dr Geoff Pearson, a senior lecturer in criminal law at Manchester University who was present during the Marseille riots of 1998 and in Charleroi at Euro 2000, said the scenes in Marseille over the weekend were the worst he had seen.
Dr Pearson has been highly critical in the past of French police, claiming that unlike other European forces they have not moved on from outdated crowd control methods which often make the situation worse.
He said: "This is developing into the most serious incident of violence and disorder that I have ever seen and that would include Charleroi 2000 and Marseille 1998. To me it was worse.
"The fact that the Russian ultras group were involved and there was disorder in the stadium. It makes it worse than I was expecting, which is quite incredible."
But French police defended themselves against accusations that their tactics were inadequate. Security experts said they relied too heavily on a massive police presence and the use of tear gas, and should have had more "spotters" mingling with crowds to call in officers to stop trouble spreading.
"I don't want anyone saying the security measures were inadequate," Laurent Nunez, the police chief said. "They were sufficient and proportionate to the risk.
"The police prevented very serious violence and damage by dispersing the most violent individuals."