Finance ministers from the world’s leading economies have warned of a “shock” to the global economy if the UK leaves the EU.
The ministers gave their opinion in a statement released at the end of a two-day meeting of G20 nations in China.
Chancellor George Osborne, who is at the event, told the BBC the referendum issue was “deadly serious”.
But UKIP’s Nigel Farage said the G20 announcement was “no surprise” as it was “mates helping each other out”.
“I’m not surprised that big government gets together to support David Cameron,” he added.
“This is big banks, big business, big governments all scratching each other’s backs, I don’t think that impresses voters.”
UK, Chinese and American officials, and Christine Lagarde – head of the International Monetary Fund (IMF) – have been discussing key issues for the global economy during the G20 summit in Shanghai.