Thousands of refugees could flock to Britain from France if voters decide to leave the European Union, a spokesman for David Cameron said on Monday, underlining that the prime minister's stance over an exit would hurt security.
In what critics said was the start of a "campaign of fear" to keep voters in the European Union, the spokesman said leaving the bloc could harm an agreement with France, which allows British border guards to make immigration checks.
"We have that relationship with the French partly because we are both partners in the EU. Should we leave the EU there is no guarantee that we would see those controls continuing," the spokesman said.
"If those controls didn't continue then as we know there are thousands of people there, who are there specifically because they want to come to the UK, who would then come to the UK ... there would be nothing to stop thousands of people crossing the Channel overnight."
Britain's border with France is on the French side of the channel, which separates the countries and at the northern port town of Calais, thousands of migrants are living in the so-called "Jungle" camp in the hope of crossing to England.
The arrangement has been criticised by some French officials, with Calais Mayor, Natacha Bouchart, calling for border controls in her town to be returned to Britain.
Cameron has thrown his weight behind a plan by European Council President, Donald Tusk, to keep Britain in the 28-member bloc, saying if agreed by other EU leaders, he would get the best deal for Britain and would campaign to stay in.
Many in his own ruling Conservative Party have accused Cameron of refusing to seriously entertain a departure from the European Union, and those campaigning to leave said that the latest warning had been choreographed to instil fear in voters.
"The prime minister is now resorting to scaremongering," said Arron Banks, the co-founder of Leave EU.
"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," he said in a statement.
Asked whether Cameron had launched “project fear,” his spokesman said that the leader was raising a genuine concern, especially since voters have repeatedly said that they wanted him to tackle with the rising migration in the EU renegotiation.