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 No 10 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 Number 10 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".