The International Monetary Fund has said Britain's shock vote to leave the European Union a week ago has created uncertainty that poses a major threat to the global economy.
"We see the uncertainty right now as probably the biggest risk to the global economy," said IMF spokesman Gerry Rice, calling on European leaders to take "decisive" actions that could lower the threat.
The EU and Britain face negotiations on the Brexit that are expected to be difficult and protracted.
EU leaders say that until Britain formally begins exit proceedings, no talks can begin - formally or informally - on resetting Britain's ties with the EU, a process meant to last two years.
British Prime Minister David Cameron, who resigned after the Brexit vote on 23 June, has resisted pressure to immediately activate the Article 50 mechanism to leave the EU, saying he is leaving it to his successor, who will not be named until 9 September.
"Brexit has created significant uncertainty and we believe this is likely to dampen growth in the near term, particularly in the UK but with repercussions also for Europe and for the world economy," said Rice in a regularly scheduled IMF news conference.
"We need to be ready, all of us policymakers, with decisive actions that can help mitigate that as much as possible."
Meanwhile, World Bank President Jim Yong Kim has given an impassioned defence of the world's leading multi-lateral institutions following the Brexit.
Speaking in New Delhi, Mr Kim said the need for multi-country co-operation was heightened "at a time when countries all over the world are looking inward and blaming their problems on other people".
Without naming Britain, Mr Kim criticised nations that he said were "trying to block their borders and trying to take themselves out of agreements with other countries".
The victory for British campaigners seeking to leave the EU, and the protectionism espoused by US presidential candidate Donald Trump, have raised fears of a pullback from the so-called Bretton Woods institutions.
The World Bank and the IMF were founded in 1945 - along with the United Nations - and led the rebuilding of the global economic system following World War II.
"The EU was one of the greatest examples of an attempt at multi-lateralism," Mr Kim said at the conclusion of a two-day visit to India, on which he met with Prime Minister Narendra Modi and other ministers to discuss deeper cooperation.
He called such institutions the "key to peace and prosperity" saying "that is how we are going to solve the problems of the world".