Eurosceptics have reacted angrily to claims that migrant camps could spring up along the coast of south east England if Britain leaves the European Union.
David Cameron is reportedly set to argue France would scrap the agreement which allows UK officials to conduct border checks on the French side of the Channel if Britain votes to withdraw from the EU in the forthcoming referendum.
The Prime Minister is preparing to make national security issues a key plank of his campaign to keep Britain in the EU over the coming weeks, The Daily Telegraph has said.
He will say that so-called "Brexit" would see France pull out of the 2003 Le Touquet treaty which means that checks for migrants trying to stow away on lorries or trains heading for Britain are carried out in Calais.
"The French would love to pull out of the arrangement," a senior source told the Telepgraph. "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."
The claim was dismissed as "scaremongering" by Vote Leave campaign chief executive Matthew Elliott.
"This is blatant scaremongering from Number 10 that has no grounding in reality," he said.
"UK border controls are in France because of a bilateral treaty, not because of our EU membership, and a result of the camps in Calais, not the cause of them. Clearly, No 10 is in a blind panic over the failing renegotiation."
The latest spat erupted amid continuing speculation as to which way some senior Conservative Party figures would vote once the referendum campaign is under way.
Justice Secretary Michael Gove – who is said to be torn between loyalty to Mr Cameron and an instinctive desire to leave – appeared to be keeping a low profile, avoiding journalists' questions, despite a major Government launch on prisons policy.
London mayor Boris Johnson – who has been courted by both camps - was also keeping observers guessing, insisting he would like to remain in a reformed bloc but would "wait until you see the whites of their eyes" before making his decision.
"This is the moment to stiffen the sinews, summon up the blood, squint down the barrel and only when you see the whites of their eyes should you finally let fly and decide whether to stay or leave the EU; because the arguments are as finely balanced as they have ever been," he wrote in his column in the Telegraph.