Finance ministers from the world's leading economies are warning of a "shock" to the global economy if the UK leaves the EU.
The ministers give their assessment in a statement released at the end of a two-day meeting of G20 nations in China.
UK Chancellor George Osborne, who is at the event, told the BBC the issue was "deadly serious".
The UK will vote on whether to leave or remain in the EU on 23 June.
UK, Chinese and American officials and the head of the IMF, Christine
Lagarde, have been discussing key issues to the global economy during a
G20 summit in Shanghai since Friday.
'Adventurous journey'
The outcome of the UK's EU referendum has been mentioned among global
risks to the world economy and is set to appear in the final
communique, according to a draft seen by BBC News.
BBC political correspondent Robin Brant said the draft was not expected to change.
Officials travelling with George Osborne say this prominent mention
of the UK's referendum in the final communique is unusual - but 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."
'Forceful'
Asked if he or his officials had asked for the warning to be included
in the statement, the chancellor said: "We've got countries around the
table like the United States of America, like the IMF, like the Chinese
who frankly don't do what anyone tells them to do and they… have come to
a unanimous verdict that a British exit from the EU would be a shock to
the world economy."
A senior official from the Treasury told the BBC the chancellor's US
counterpart raised the issue during the Shanghai meeting, as did others
including senior Chinese officials and the head of the International
Monetary Fund.
IMF chief Christine Lagarde was said to have been "forceful" when she
mentioned it over a dinner meeting on Friday, our correspondent said.
Meanwhile, London Mayor Boris Johnson has clarified his position on
the idea that a vote to leave the EU could force Brussels to give
Britain a better deal and trigger a second referendum.
Johnson had previously suggested that only by voting to leave would the UK "get the change we need".
But in The Times, Johnson stated categorically: "Out is out".
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.
Cameron is visiting two businesses in County Antrim.
