IMF boss Christine Lagarde, unveiling the global lender's latest health check on the British economy just six weeks before Britain votes on whether to remain in the EU, added that Brexit could push the country into recession, echoing comments from Bank of England (BoE) chief Mark Carney.
The latest warning comes as Prime Minister David Cameron campaigns fervently to keep Britain in the 28-nation EU in a referendum on June 23.
Leave supporters, which hit out against Carney for his and the BoE's stance on Thursday, also criticised the IMF's intervention.
Lagarde admitted today that sometimes the IMF is wrong.
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"We are one of the very few institutions that actually acknowledge when we are wrong... But on that particular one which relates to the negative consequences of Leave vote, we have looked very carefully at the whole range of existing opinions (and) calculations."
Opinion polls are showing that the nation is still largely undecided.
Lagarde, speaking at the Treasury in central London, told reporters that the IMF's findings were not politically motivated.
"We are doing it because it's a significant downside risk, number one. Second, it's not just a domestic issue... it's an international issue.
"I don't think that in the last six months I have visited a country anywhere in the world where I have not been asked 'what will be the economic consequences of Brexit?'."
The IMF meanwhile forecast the British economy would rebound in the second half of this year if the country stays in the EU.
"Assuming that... The UK voters choose to remain... We will expect growth to rebound," Lagarde said.
Questioned about Carney's comments, Lagarde told reporters: "A technical recession is one of the probabilities in the downside scenario in case of a Leave vote."
The IMF added that Brexit would spark fresh markets volatility and a lengthy period of uncertainty.