France has warned 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 a non-EU Britain and stay in the bloc.
French Economy Minister Emmanuel Macron echoed comments by 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, 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," Macron told the Financial Times newspaper on Thursday, 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.
Macron's comments, which support Cameron's argument that an EU exit after the June 23 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.
A British exit from the EU would rock the EU - already shaken by differences over migration and by fragility within the euro zone - by ripping away its second-largest economy, one of its top two military powers and by far its richest financial centre.
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."
More than three-quarters of manufacturers and traders in Britain's car industry believe that staying in the European Union is best for their business because of the risk of trade barriers and skill shortages if the country leaves, a poll showed on Thursday.
The British car industry body SMMT said 77 per cent of its members said remaining in the EU was the best option when the country votes in a referendum on June 23, according to a survey conducted for the body by polling firm ComRes.
The prospect of leaving the EU also drove growth in Britain's dominant services sector to a near three-year low last month, according to an economic survey.
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," Johnson wrote in The Sun newspaper.
"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," he said.
