LONDON - The migrant crisis in Europe is fueling debate in Britain ahead of a referendum on whether to stay in the European Union. 

Prime Minister David Cameron has warned that leaving the EU could lead to thousands more migrants arriving in the country.  There is growing tension in the French port of Calais, where thousands of migrants are living in squalid camps.

The graffiti on an underpass in Calais’ so-called Jungle Camp reads, "London Calling," the prime target for the 4,000 migrants who live here in makeshift tents and shipping containers, waiting for their chance to somehow get to Britain.

Under a 2003 agreement, British border checks are performed on French soil in Calais, which keeps the migrants in France.

Speaking Monday, British Prime Minister David Cameron warned that the deal would be under threat if Britons voted to leave the union.

“There are a lot of opposition politicians in France who would love an excuse to tear up that treaty and would like the border not to be in France, but to be in Britain and I do not want to give people an excuse to do that," said Cameron.

Border police and migrants play a constant game of cat and mouse in Calais port.  Migrants try to sneak into trucks as they board ferries bound for Britain or even break into the tunnel that carries trains beneath the English Channel.

There is growing tension between migrants and locals.  Members of the anti-Islam group ‘PEGIDA,’ which was founded in Germany, attempted to stage a march Saturday, despite local authorities banning all public protests in Calais.

North France region President Xavier Bertrand welcomed the ban.

“These people do not want the common good; they just want to exploit poverty and anger and they have no place in Calais," said  Bertrand.

In recent weeks, the migrant camps have been hit by snow and winter storms.  Natalie Roberts of Doctors Without Borders, the only major charity working around Calais, says conditions at one camp near the town of Dunkirk desperately need to be improved.

“There are about 2,500 people living there at the moment, mainly Kurds from Iraq and from Syria.  And the conditions in that camp are really horrific.  People are living in this kind of muddy wasteland.  Every time it rains, the mud goes up to your knees.  There are rats; the people are getting scabies because of poor hygiene," said Roberts.

Doctors without Borders successfully lobbied for a new camp to be built in Dunkirk, which is due to open later this month.

The longer-term fate of the camps, and of their residents, could depend on Britain’s choice to stay in or leave the European Union.