Britain would become a minor trading post no more important on the world stage than the island of Guernsey if it voted to leave the European Union next week, France's economy minister has been quoted as saying.
In an interview with French newspaper Le Monde, Emmanuel Macron said the EU would also have to send "a very firm message and timetable" to Britain if its voters backed quitting the EU, known as Brexit, in a referendum on Thursday.
"In the interests of the EU, we can't leave any margin of ambiguity or let too much time go by," Macron was quoted as saying. "You're either in or you're out."
"Leaving the EU would mean the 'Guernseyfication' of the UK, which would then be a little country on the world scale. It would isolate itself and become a trading post and arbitration place at Europe's border."
Guernsey is a tiny island in the Channel between Britain and France.
The International Monetary Fund has said a vote to quit the EU could leave Britain's economy more than 5 percent smaller by 2019 than if it stays in the 28-nation club.
Former Italian prime minister Mario Monti also had a severe message for the British government, saying holding the referendum at all was "highly irresponsible".
"The British government and Prime Minister (David) Cameron decided to risk the European Union dissolving, basically to strengthen the prime minister's position within his own party," Monti said.
Speaking to SkyTG24 television at a conference in Venice, Monti said the British government's actions had also raised the risk of Scotland leaving the United Kingdom.
Opinion polls this week suggested the "Leave" camp was ahead of the "Remain" side ahead of the June 23 vote.
Campaigning was suspended after the murder of British parliamentarian Jo Cox on Thursday.