British traffickers are involved in smuggling migrants into the UK from northern France, a mayor has alleged.
One migrant, who said he had spent £15,000 so far on trying to reach the UK from Syria, also said many of the criminals were British nationals.
The BBC saw cars with UK number plates at a camp on Dunkirk's outskirts.
The mayor of nearby Teteghem, Franck Dhersin, said UK-registered cars - driven by what he called English mafia - were often spotted there.
'So, so scared'
He said people in the UK who complained about migrants trying to reach the country did not realise "the cars are English and the owners are English".
The tactics of migrants around Dunkirk are different from those in Calais, where hundreds try to board lorries and scale fences every night to gain entry to the Channel Tunnel.
The Dunkirk migrants are driven to locations where they can try to stow away on lorries.
The Syrian migrant who spoke to the BBC on condition of anonymity said the £15,000 he had paid traffickers was all of his money.
He has spent £12,000 on getting from Syria to France and a further £3,000 on trying to cross the Channel.
He said migrants were threatened by traffickers if they did not follow orders.
"I'm so, so scared," he said.
"They have guns. They put it to someone's head and said 'You move again and I will kill you'."
It's a similar story at the Teteghem camp a few miles away.
Mr Dhersin said the traffickers were "very violent" and "just want to make money with immigrants".
He said British traffickers had been operating in the area for several years, but were now much more organised.
Until Tuesday he had insisted on visiting the Teteghem camp despite the dangers.
"I wanted to show that a French mayor can go where he wants," he said.
"I said 'I will enter and if you don't want [me to], I will come back with a policeman'."
But Mr Dhersin said that a man there had shown him a gun and threatened to rape a reporter he was with.
Police have advised him not to return.
Pascal Aerts, who leads the officers policing migrants in Calais, has told the BBC he is aware of the probable involvement of British people.
"I don't have proof that they're English but we know perfectly there do exist links between the traffickers and the receivers in Great Britain, and with the traffickers who work in France."