Leaving the European Union would make the UK as significant as Guernsey, France's economy minister has said.
Emmanuel Macron told Le Monde newspaper that Britain would become "a little country on the world scale [that] would isolate itself... at Europe's border".
He said the EU should send "a very firm message" about the consequences of a British vote to leave the bloc.
Vote Leave said French ministers wanted Britain to stay in the EU and carry on paying millions to it.
Mr Macron's comments were published after remarks by Russian President Vladimir Putin, who suggested Prime Minister David Cameron, who favours staying in the EU, may have called the referendum to "blackmail" or "scare" Europe.
The Russian leader would not say whether he wanted Britain to leave the EU and said voters faced a "complicated" choice.
The UK votes next Thursday in an in-out referendum.
In other developments:
- The IMF has said Brexit could mean the UK misses out on up to 5.6% of GDP growth by 2019
- Field Marshal Lord Guthrie, a former chief of defence staff, has switched sides to Leave, saying he is worried by the prospect of "a European army"
- The Times newspaper has said it supports Remain
- The Remain and Leave groups have suspended their campaigning until Sunday following MP Jo Cox's death
Mr Macron told Le Monde: "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."
And he said the European Council would have to deliver an ultimatum to the UK about its intentions and that France's President Hollande would be very clear.
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"If the UK wants a treaty of commercial access to the European market, the British will have to contribute to the European budget like the Norwegians or the Swiss. If London doesn't want that, then the exit will have to be total," Mr Macron said.
'New trade deals'
In other comments, made to French radio, Mr Macron said the British referendum marked the end of an era for the EU.
"I believe in Europe, but in its reorganisation," he told RTL radio. "It's the end of an ultra-liberal Europe that has lost its political direction. The European project cannot only be a system of abolishing rules."
He said the debate in Britain was about correcting the effect of ultra-liberal policies "that they pushed us into".
In the event of Brexit, Europe should "act fast to avoid other countries starting a similar process", he said.
"There must be no question of Denmark, the Netherlands, Poland, deciding they want the same status."
Vote Leave said Mr Macron's comment "sums up the Remain campaign", which it said had "nothing positive to say about Britain".
A spokesman added: "Of course French ministers want Britain to stick in the EU so we can continue to hand Brussels £350m every week.
"Instead of subsidising French farmers we should take back control and spend our money on our priorities like the NHS."
Meanwhile, the IMF said Brexit was the "largest near-term risk" to the UK economy, in its annual report on the UK's economic outlook.
It added that the net economic effects would probably be "negative and substantial".
But campaign group Economists for Brexit said the consensus that a UK exit would be bad for the economy was "based on flawed EU-centric models".