France warned Britain on Thursday morning it would end border controls and let thousands of migrants move on to Britain if voters backed leaving the European Union.
It also said it would open its arms to British-based banks wanting to flee an non-EU Britain and stay in the bloc.
French economy minister Emmanuel Macron echoed comments by British prime minister David Cameron that a migrant camp known as the "Jungle" in the northern French coastal town of Calais could move to southern England in the event of a British EU exit.
Speaking ahead of a Anglo-French security summit in Amiens, Mr Macron said a British exit would scupper a border deal that halts migrants in France, but that Paris would be happy to accept bankers fleeing London.
"The day this relationship unravels, migrants will no longer be in Calais," Mr Macron told the Financial Times newspaper, adding that rules allowing British-based banks to operate across the EU would be lost.
“Collective energy would be spent on unwinding existing links, not re-creating new ones,” he said, a comment aimed at the view of British eurosceptics that a new deal could be made.
Mr Macron’s comments, which support Mr Cameron’s argument that an EU exit after the June 23th referendum could undermine security, led television news reports in Britain, where opinion polls indicate immigration is the biggest concern for voters.
Opponents of membership said the comment was part of a campaign to scare British voters into supporting membership.
In a move that underscored big company concern over the impact of a possible British exit, Germany's BMW wrote to British employees who make its luxury Rolls-Royce car about the risks of a Brexit, as leaving is known.
“As a wholly-owned BMW Group company, it is important for all Rolls-Royce Motor Cars employees to understand the view of our parent company,” BMW said in the letter. “We believe it’s much better to be sat at the table when regulations are set and have a hand in their creation, rather than simply having to accept them.”
Opponents of EU membership, including Cameron's main Conservative party rival, London Mayor Boris Johnson, said the British people were being fed scare stories in an attempt to garner support for the EU.
"Let's believe in ourselves again, rather than clutching the skirts of Brussels, " wrote Mr Johnson. "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."
– Reuters