France could end UK border controls in Calais and allow migrants to cross the Channel unchecked, if the UK leaves the EU, France’s finance minister has said.
Emmanuel Macron told the Financial Times his country could also limit access to the single market and try to tempt London’s bankers to relocate.
His comments come as David Cameron and Francois Hollande prepare for security and migration talks in France.
Pro-exit London Mayor Boris Johnson urged voters to “ignore scaremongers”.
British voters will be asked whether the UK should remain in the European Union or leave, in a referendum on Thursday 23 June.
Mr Macron told the FT that if the vote was in favour of a so-called Brexit, it could bring to an end the agreement between the two countries that allows the UK to conduct border controls on the French side of the Channel.
‘Bankers’ red carpet’
There are currently believed to be about 4,000 migrants amassed in Calais, hoping to cross to the UK.
“The day this relationship unravels, migrants will no longer be in Calais,” Mr Macron told the newspaper, adding that France would also roll out a “red carpet” to London’s bankers if the UK voted to leave the EU.
The migration crisis is among topics expected to be discussed when the UK prime minister and French president meet in Amiens, northern France.
Mr Cameron has also previously claimed migrant camps could move to England if the UK left the EU – although his comments were dismissed as “scaremongering” by those campaigning for an EU exit, including his former Defence Secretary Liam Fox.
On Monday, there were clashes as French demolition teams dismantled huts in the part of the Calais migrant camp known as the Jungle.
by BBC political correspondent Ben Wright
Into the already febrile referendum debate in Britain walks France – making it clear there’ll be no special treatment for the UK if it votes to leave the EU in June. Downing Street is likely to welcome this warning from Emmanuel Macron.
This annual summit will begin with a commemoration at a British war cemetery to mark the 100th anniversary of the Battle of the Somme, but the talks between David Cameron and Francois Hollande will be framed by the UK’s referendum campaign.
National security is now frequently cited by the government as a vital reason for remaining in the EU and the summit is a chance to hammer that message home. The foreign, home and defence secretaries will meet their French counterparts here too.
But Leave campaigners have said free movement rules leave Britain more exposed to a Paris-style attack.
The two leaders are expected to emphasise their “relentless” determination to tackle extremism and terrorism at what is the first UK-France summit since last year’s attacks in Paris, which left 130 people dead and hundreds wounded.
Mr Cameron and Mr Hollande will commit to intensifying police and security co-operations and are expected to announce a 1.5bn investment in a new phase of building advanced drones.
They will warn against an EU exit, saying membership offers more “security and greater capacity to project power”.
Read more on the UK’s EU referendum
Referendum timeline: What will happen when?
The view from Europe: What’s in it for the others?
Speaking ahead of the summit, Mr Cameron, who is calling for the UK to stay in a reformed EU, said Britain and France “are proud allies”.
He added: “I am convinced that the UK’s membership of the EU gives us greater security and greater capacity to project power globally.
“In an ever-more uncertain world, we gain from our membership of these international organisations.”
‘Lift our eyes’
Meanwhile, writing in the Sun, Boris Johnson said it was time for Britain to “break free” from the EU.
He wrote: “Let us believe in ourselves again, rather than clutching the skirts of Brussels.
“Let us lift our eyes to the horizon and take a once-in-a-lifetime opportunity. Ignore the scaremongers, we are bigger, better and greater than they pretend.”
Vote Leave, one of the groups campaigning for an EU exit, accused Downing Street of “invoking every made up scare story it can come out with to scare the public”.
What are the arrangements with France?
The ‘Treaty of Le Touquet’ is an agreement between France and the UK that has been in place since 2003 and governs border control arrangements and immigration.
The treaty ensures that immigration checks are carried out before passengers embark on cross-Channel services.
French border police have immigration checkpoints at Dover, while the UK has immigration checkpoints at Calais and Dunkirk. But at Calais and Dunkirk, passengers also go through French exit checks, as well as UK immigration entry checks.
In theory this stops those seeking to reach the UK from doing so without their immigration status being checked first, but this has led to the establishment of camps in Calais such as the so-called Jungle and previously, Sangatte.
Read more: www.bbc.co.uk