Francois Hollande has warned there will be “consequences” for the way France seeks to control immigration into Britain if the UK votes to leave the European Union.
Following a UK-France summit in Amiens, the French president said Britain could not escape the fact that relations between the two countries would change in the event of a vote for Brexit.
While he stopped short of repeating a warning by the French finance minister that UK border controls in Calais could be withdrawn - potentially opening the door for many more migrants to cross the Channel - he left little doubt the issue would be on the table.
David Cameron dismissed suggestions that the French intervention was part of a giant “David Icke-style” conspiracy to “stitch-up” the result of the referendum on Britain's EU membership as “nonsense”.
The Prime Minister stressed, however, that it was important that voters listened to what friendly countries such as France were saying when they came to decide.
Mr Hollande denied that he was trying to “scare” UK voters, insisting such tactics would be counterproductive.
However, he made clear that it would not just be immigration issues that would be affected - pointing also to the European single market and the financial services sector.
He acknowledged, nevertheless, it was the situation at Calais - where hundreds of migrants are camped in the Jungle while they try to find a way across the Channel - that was the most sensitive issue.
“I don't want to scare you but I just want to say the truth. There will be consequences in many areas,” he said.
What has the EU ever done for us?
What has the EU ever done for us?
1/7 1. It gives you freedom to live, work and retire anywhere in Europe
As a member of the EU, UK citizens benefit from freedom of movement across the continent. Considered one of the so-called four pillars of the European Union, this freedom allows all EU citizens to live, work and travel in other member states.
2/7 2. It sustains millions of jobs
A report by the Centre for Economics and Business Research, released in October 2015, suggested 3.1 million British jobs were linked to the UK’s exports to the EU.
3/7 3. Your holiday is much easier - and safer
Freedom to travel is one of the most exercised benefits of EU membership, with Britons having made 31 million visits to the EU in 2014 alone. But a lot of the benefits of being an EU citizen are either taken for granted or go unnoticed.
4/7 4. It means you're less likely to get ripped off
Consumer protection is a key benefit of the EU’s single market, and ensures members of the British public receive equal consumer rights when shopping anywhere in Europe.
5/7 5. It offers greater protection from terrorists, paedophiles, people traffickers and cyber-crime
Another example of a lesser-known advantage of EU membership is the benefit of cross-country coordination and cooperation in the fight against crime.
6/7 6. Our businesses depend on it
According to 71% of all members of the Confederation of British Influence (CBI), and 67 per cent of small and medium-sized enterprises (SMEs), the EU has had an overall positive impact on their business.
7/7 7. We have greater influence
Robin Niblett, Director of think-tank Chatham House, stated in a report published last year: “For a mid-sized country like the UK, which will never again be economically dominant either globally or regionally, and whose diplomatic and military resources are declining in relative terms, being a major player in a strong regional institution can offer a critical lever for international influence.
“There will be consequences, especially in the way we handle the situation we have just mentioned in terms of immigration. There will be consequences, there is no solution where there is no consequences.”
Mr Cameron said that such advice from an ally should be treated seriously.
“When you have ministers in other governments warning about potential consequences that might happen that would be injurious to the United Kingdom, I would say listen to those things,” he said.
The Prime Minister announced £17 million of funding for enhanced security in Calais, as well as moves to relocate migrants from the port to facilities elsewhere in France.
“I applaud the action that the French government is taking to deal with the situation with the camps in Calais and to say to people that they should be seeking asylum in France, and if they are not asylum seekers they should be returned to the countries from which they came,” he said.
“That I think is absolutely the right approach and the president has my 100% backing in the work he is doing to deliver that. And as the money that we announced today shows, we are absolutely working in a joint endeavour.”
Brexit campaigners poured scorn on the suggestions from the French government that The Jungle migrant camp could be recreated across the Channel.
Before the comments from Mr Hollande, French finance minister Emmanuel Macron told the Financial Times: “The day this relationship unravels, migrants will no longer be in Calais.”
Mr Macron's comments were dismissed as “scaremongering” by London mayor and leading advocate of Brexit Boris Johnson.
Breaking into Franglais during a visit to south London, Mr Johnson told the Press Association: “I would say 'Donnez-moi un break'. There's absolutely no reason why that treaty should be changed.
“It was an inter-governmental treaty, it was the Le Touquet treaty. It was signed between the British government and the French government.
“It's not in the French interests to want to do that and it's just the usual flapping and scaremongering.”
Grassroots Out spokesman and former shadow home secretary David Davis described Mr Macron's comment as “more like bluster than a real threat”.
The Tory MP said: “The simple point is that if we leave the EU, we regain control of our borders and we decide who comes in and who doesn't. If the French start putting illegal immigrants on a train or ferry and send them to Britain, we will send them straight back to France.”
Senior Conservative Bernard Jenkin told the BBC Radio 4 Today programme: “That is the genuine line to take from the French government. What we are having now is propaganda being produced by other European governments at the request of the Prime Minister to try to scare people (out of) voting Leave.
“I don't think responsible European governments are going to cut off their noses to spite their faces just because we vote to leave. It is obviously the safer thing for the UK to take back control over our borders, over our laws, over the money we send to the EU because then we can control our relations with our European partners.”
The Amiens summit also saw an agreement to jointly invest £1.5 billion to develop the next generation of unmanned combat air systems which Mr Cameron said will be “the most advanced of its kind in Europe, securing high-end engineering jobs and expertise” in both the UK and France.