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 government all scratching each other’s backs, I don’t think that impresses voters.”
Officials travelling with Mr Osborne say this prominent mention of the UK’s referendum in the G20 meeting’s final communique is unusual. But Mr Osborne has denied that he pushed for it.
The chancellor told the BBC: “The financial leaders of the world’s biggest countries have given their unanimous verdict and they say that a British exit from the EU would be a shock to the world economy – and if it’s a shock to the world economy imagine what it would do to Britain.”
He added: “This isn’t some adventurous journey into the unknown, with all the humour attached to it, this is deadly serious.”
Elsewhere, Prime Minister David Cameron has arrived in Northern Ireland as part of his UK tour to persuade voters that membership of a reformed EU is in their best interests.