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"
What the leave and remain sides are saying about trade and the economy in the #EUref campaign
About half of UK overseas trade is conducted with the EU
The EU single market allows the free movement of goods, services, capital and workers
Trade negotiations with other parts of the world are conducted by the EU, not individual member states
UK companies would be freed from the burden of EU regulation
Trade with EU countries would continue because we import more from them than we export to them
Britain would be able to negotiate its own trade deals with other countries
Brexit would cause an economic shock and growth would be slower
As a share of exports Britain is more dependent on the rest of the EU than they are on us
The UK would still have to apply EU rules to retain access to the single market
EU referendum issues guide: Explore the argumentshttp://www.bbc.co.uk/news/uk-politics-eu-referendum-36027205http://www.bbc.co.uk/news/special/2016/newsspec_13606/content/iframe/english/index.inc.app.htmlExplore all the issuesChoose an issue:What both sides are sayingAll issuesMain viewsShare this pageEU referendum issues guideWhat the leave and remain sides are saying in the #EUref campaign
"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."