DAVID Cameron has warned there is "any number" of French politicians who would take the chance to scrap cross-Channel border controls on migrants trying to reach the UK if Britain voted to leave the European Union.
Eurosceptics reacted with fury to claim by No10 that France could tear up an agreement which means checks on lorries and trains heading for Britain are carried on the French side of the Channel – leaving thousands of migrants in the Calais "Jungle" free to cross to the UK.
However, Mr Cameron hit back, insisting there were real risks to Britain's ability to control the flow of migrants into the UK if the country voted for "Brexit" in the forthcoming referendum.
"There are any number of opposition politicians in France who would love to tear up the excellent agreement we have with France to make sure that we have our borders on their side of the Channel," he said, during a question and answer session following a keynote speech on prison reform.
"I don't think we should give those politicians any excuse to do that."
Earlier former defence secretary Liam Fox said he was "sad and disappointed to see our prime minister stoop to this level of scaremongering", while fellow MP Sarah Wollaston said Mr Cameron was "taking voters for fools".
And senior Tory David Davis said: "As the argument slips away from the Remain campaign, they are forced to rely on desperate scaremongering."
But a No10 spokesman said it was a "perfectly feasible scenario" that quitting the EU could result in Paris ending cooperation and "thousands of asylum seekers pitching up in south-east England effectively overnight".
The row broke out after The Daily Telegraph reported that Mr Cameron would argue that France could 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 – if Britain withdrew from Europe .
"The French would love to pull out of the arrangement," a senior source told the Telegraph. "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 No10 spokesman said there was "no guarantee" it would continue as it was founded on friendly working relations with Paris and there was a risk that a vote to leave the EU would "throw that whole relationship into question".
He said he could not rule out the suggestion the influx would be so great and sudden that it would result in similar camps being created in port towns such as Folkestone.
Downing Street's position was backed by Rob Whiteman, a former chief executive of the UK Border Agency, who said Mr Cameron was entitled to argue that France would "almost certainly" 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 the BBC Radio 4 Today programme.
"If you're found in a concealed vehicle, you're a clandestine, once you set foot on British soil you can claim asylum whereas if you're found in a vehicle on French soil you can't claim asylum in 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, let's say, another 50,000 asylum claims a year which we used to get before the treaty came in."
