Ministers from the world`s biggest economies lined up on Saturday to warn against a potential British exit from the European Union.
A so-called Brexit would would be a "shock" that ranks among rising downside risks and vulnerabilities for the world economy, the G20 finance ministers and central bank chiefs said after a meeting in China.
The issue, which will be put to British voters in June, was cited in the seventh line of a communique, highlighting its importance to the officials in Shanghai.
The chances of a vote to leave are seen as having risen after the Mayor of London Boris Johnson backed the cause, pitting himself against Prime Minster David Cameron, long a rival within the ruling Conservative Party.
In Shanghai, IMF managing director Christine Lagarde told reporters the issue was included in the G20 communique "as soon as the meetings really effectively started".
US Treasury Secretary Jacob Lew emphatically backed a vote to stay in the EU.
"Our view is that it`s in the national security and economic security of the United Kingdom and European Union and of the United States for the United Kingdom to stay in the European Union," he said, adding that meant a "more secure world".
French Finance Minister Michel Sapin added that no real debate had been necessary on the issue.
Britain`s Finance Minister George Osborne said the question was "deadly serious" and not "some adventurous journey into the unknown".
"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," he told the BBC. "If it`s a shock to the world economy imagine what it would do to Britain."
Johnson, who has a rare ability to appeal to voters outside his own party in Britain, came out fighting on Saturday for a vote to leave, saying that the country had "given away control of our destiny".