David Cameron's warnings that France could scrap a border deal if Britain leaves the EU have been backed by a former national security adviser.
Sir Peter Ricketts, who was the UK's ambassador to Paris for four years until last month, said a rethink would be "very likely" and dismissed Eurosceptic accusations the Prime Minister was "scaremongering".
Downing Street prompted a furious backlash from pro-Brexit campaigners by claiming a "leave" vote in the referendum could free thousands of migrants in the Calais "Jungle" to travel to the UK and potentially create tent cities in Kent.
Mr Cameron raised the prospect that EU withdrawal would jeopardise not only the 2003 Le Touquet agreement requiring checks on lorries and trains on French soil, but also other cross-border co-operation on terrorism and organised crime.
Senior Conservatives including ex-defence secretary Liam Fox accused the Prime Minister of misleading voters, pointing out that the agreement was between the two countries and not an EU measure.
But the PM received the firm backing of Sir Peter, who told BBC Radio 4's Today: "This is a bilateral treaty but it was made in a multilateral context where Britain and France are working very closely together across a whole range of issues in the interior, justice area, police cooperation and so on.
"If the context changed, and Britain made a major decision to leave the EU, then I think it is very likely that the French would review its position as well.
"It has 1,000 of its crack riot police deployed in Calais, far more than in Marseille. They are bottling up effectively 7,000 or more migrants in the camps.
"They are taking a lot of political flak for that, there is a lot of humanitarian pressure on them.
"They are doing it effectively to protect our border. We get a secure border; the French carry a lot of the load.
"They are doing it because they see us as a very important ally in the EU on a whole range of areas of cooperation on police and crime. If that stopped, then the incentives change for France."