Brexit could lead to asylum seekers setting up camps in the UK, warns Downing Street
Thousands of asylum seekers could camp on British soil if the UK quits the European Union, Downing Street has claimed.
Mr Cameron is to warn that a Brexit would lead to camps of illegal immigrants in the UK because France could stop allowing UK officials to make their immigration checks on the French side of the border.
Raising the prospect of the Calais Jungle moving to south-east England, a spokesman for the Prime Minister said: "It's our position that we have a positive and strong working relationship with the French at the moment, but clearly the point that is being made here is that should we leave the EU then some of these other arrangements that we may have with other countries, for example the juxtaposed controls we have with France, could be called into question.
"The point here is that if that's called into question and those controls cease to exist, then you have potentially thousands of asylum seekers camped out in northern France who could be here almost overnight."
Under the 2003 Le Touquet treaty between the UK and France, Britain is allowed to conduct border controls at French rather than UK borders - meaning it checks for migrants stowing away on lorries or trains bound for Britain in Calais, not Dover.
Britain will face an influx of more than 50,000 asylum seekers a year if it leaves the EU, the former head of the UK Border Agency has also suggested.
Rob Whiteman, former chief executive of the UKBA, warned: "The bilateral treaty takes a lot of work on the French side to maintain and I think it is almost certain that if we did leave the EU the treaty would come to an end. It is not a foregone conclusion but I think it is fair for the Prime Minister to claim that the French would almost certainly bring it to an end.
"There's been lots of upsides for the UK since the treaty was negotiated in 2003, not much upside for the French. The reason we wanted the treaty is that if you are found in a concealed vehicle, you're a clandestine, once you've set foot on British soil you can claim asylum whereas if you are found concealed in a vehicle on French soil you can't claim asylum for Britain. Before that treaty was put in place, asylum claims were running at 80,000 a year in the UK. They are now running at about 30,000 a year. So we would probably see another 50,000 asylum claims a year.
"At the time the French felt there would be an upside for them in that if it was clear that people couldn't easily get to Britain it would stop the Sangatte building up again but history has shown that not to be the case. The French authorities still have a huge amount of pressure on their side and now 'The Jungle' as it is known, has developed."
The Prime Minister is to argue that a Brexit would also leave Britain vulnerable to terror attacks and that migrant camps will spring up across the south-east of England.
Mr Cameron will make national security issues the centrepiece of his campaign to keep Britain in the European Union.
It is understood he will set out the argument in the days and weeks after he calls the date of the referendum, which is expected to be set later this month.
Alongside Theresa May, the Home Secretary who has now indicated that she will join the "Remain" campaign, Mr Cameron (right) will warn that being part of the EU allows Britain to share crucial information that could prevent a Paris-style attack on the streets of Britain.
Crucially, Downing Street hopes to use the migration crisis on the continent to its advantage by warning that a Brexit would lead to camps of illegal immigrants in the UK.
Mr Cameron will warn that if Britain leaves the EU, France will stop allowing UK officials to make the checks.
"The French would love to pull out of the arrangement," a senior source said. "We will be telling people - look, if we leave the EU the Jungle camp in Calais will move to Folkestone. That is not something people want."
"Yes, this will be a debate about migration and the economic benefits of the EU. But security is going to play just as big a part. These are dangerous times and we will make the argument that Britain is much safer as part of the European Union. There is a compelling case to be made"
The Prime Minister has been accused of scaremongering by eurosceptics who are highly critical of the deal he has done with Brussels.
Arron Banks, Leave.EU co-founder, said: "Having failed to deliver the fundamental reform of the European Union that he promised in his Bloomberg speech, the Prime Minister is now resorting to scaremongering.
"The agreement we have to process migrants in Calais is with France, not the EU. There is no reason for this to change on leaving the EU.
"The key issue here is that we should not have to accept migrants that have arrived from a safe country.
"The scandal of countries effectively waving migrants through Western Europe to our shores will only end when we are able to control our borders again."
David Davis, the Eurosceptic Tory MP, said: "As the argument slips away from the Remain campaign, they are forced to rely on desperate scaremongering.
"We already have a process where air carriers transporting passengers with no visa are fined as well as being responsible for returning people they have flown to the country illegally.
"There is no reason why the same policy would not work for trains and ferries." (© Daily Telegraph, London)