All of the world's biggest countries have united in warning that a British exit from the European Union would provoke "a shock to the world economy", George Osborne said.
The Chancellor hailed the unanimous agreement on the risk of Brexit after it was included in a communique after a meeting of G20 finance ministers in Shanghai, China.
He said: “Finance leaders and central bank governors of the world's biggest economies have raised serious concerns about the risks posed by a UK exit from the EU.
“They have concluded unanimously today that what they call the shock of a potential UK vote to leave is among the biggest economic dangers this year.
“If that's their assessment of the impact on the world economy, imagine what it would do to the UK.”
After securing the public backing of colleagues from major nations including China and the US for the Government's pro-membership stance, he also took a thinly veiled swipe at Boris Johnson.
The Mayor of London faced accusations of “flip-flopping” after he ruled out the possibility of a vote to leave on June 23 leading to a better offer from Brussels and a second referendum.
In his declaration of support for the leave campaign last week, Mr Johnson wrote in his Telegraph column that “EU history shows that they only really listen to a population when it says 'no”'.
It was widely seen as backing for the idea of a fresh vote - also backed by former Tory leader Michael Howard and considered a valuable weapon for winning over wavering voters.
Mr Johnson has now told The Times that he was not in favour of using a popular rejection of EU membership as leverage to secure further reforms - saying “out means out”.
“What I want is to get out and then negotiate a series of trade arrangements around the world,” he said.
Prime Minister David Cameron, who is in Northern Ireland on the latest leg of a tour taking the pro-EU message to all parts of the UK, has dismissed the idea as “complete fiction”.
He says he would immediately trigger the process of pulling out if the country voted for it.
“Many of those who want to ask you to vote to leave actually are not really sure whether they want to leave or not. I think that should set alarm bells going in the minds of voters about the potential dangers and risks that we could face if we leave,” the PM said, in a fresh swipe at his opponents.
In an apparent renewal of hostilities with Mr Johnson - seen as his closest rival for the party leadership when Mr Cameron steps down - Mr Osborne said: “This isn't some amusing adventure into the unknown. A British exit would hurt people's jobs, livelihoods and living standards - it's deadly serious.
“It's my responsibility as Chancellor to make it clear to people what the economic risks are - and that we are stronger, safer and better off remaining in a reformed EU.”