London: A British EU exit could mean thousands of migrants landing on Britain's shores "overnight", the government said on Monday, stepping up the rhetoric in the campaign ahead of a referendum on membership.
Prime Minister David Cameron's spokesman said a "Brexit" could undermine a Franco-British bilateral agreement that allows Britain to carry out border checks on French soil, stopping many migrants.
"Should Britain leave the EU there's no guarantee those controls would remain in place," Cameron's spokesman told reporters at a daily briefing.
"If those controls weren't in place there would be nothing to stop thousands of people crossing the Channel overnight and arriving in Kent (southeast England) and claiming asylum," he said.
The Le Touquet border treaty was signed in 2003 by the then British and French interior ministers David Blunkett and Nicolas Sarkozy following a series of riots at the Sangatte migrant camp near Calais.
The treaty allowed for joint British and French border controls in Channel ports in France, easing the pressure on Britain's border force that came from the thousands of migrants from Sangatte who were making daily attempts to board Channel Tunnel trains.
"There are any number of opposition politicians in France who would love to tear up the excellent agreement we have with France," Cameron said after a speech later in the day. "I don't think we should give those politicians any excuse to do that," he added.
Many in northern France believe that getting rid of British border controls in France would mean the end of controversial migrant camps like the "Jungle" as well as easing the burden on French police.
Xavier Bertrand, the newly-elected head of the northern French region that includes the port of Calais, said the British government's comments meant that revising the treaty was "no longer taboo".
Bertrand, from the opposition Republicans party, hailed the comments as "a turning point". If Britain were to leave the European Union "they would automatically take the border back. If they stay, we cannot remain as we are," he added.
'Scaremongering'?
The warning about migrants arriving in Kent, a picturesque area known as the "Garden of England" for its fruit and hop growing was described as a political move by Cameron ahead of the referendum.
"Arguments to this effect are likely to play a significant role in the campaign, on both sides of the referendum question, said Marley Morris of think tank IPPR.
"But our research suggests that claims that EU migration will remain high or increase in the event of Brexit do not wash with the public."
Cameron's words were quickly contested by Britain's anti-EU campaigners, who said the border deal would not be affected by the country's EU membership status.
Arron Banks, co-founder of the Leave.EU campaign, accused Cameron of "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," he said.
Immigration is the central issue for many Britons in the debate over whether Britain should stay or leave the European Union ahead of an in-or-out referendum expected to be held later this year.
Recent opinion polls indicate that more Britons want to leave than want to stay and experts say attitudes have hardened in reaction to Europe's migrant crisis.
Rob Whiteman, a former head of Britain's Border Agency, supported Cameron by saying Brexit would mean "almost certainly" that France would end the treaty.
"There has been lots of upsides for the UK since the treaty was negotiated in 2003, not much upside for the French," he told BBC radio.
"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, let's say, another 50,000 asylum claims a year which we used to get before the treaty came in."
