Brexit could push the United Kingdom into recession, the International Monetary Fund (IMF) has warned.
Christine Lagarde, the director of the IMF, said today the impact of the UK leaving the EU ranged from "pretty bad to very, very bad".
Lagarde's intervention will be seen as a boost for David Cameron, George Osborne and the 'Remain' campaign.
The prime minister and chancellor have repeatedly warned of the economic risks of Brexit.
The IMF also revealed it will publish a second report into the impact of Brexit just days before polling day.
The pro-Brexit campaign has hit back, accusing the IMF of "blood-curling" forecasts.
Speaking alongside the chancellor in the Treasury today, Lagarde rejected the suggestion Osborne had a role in the IMF's conclusions about Brexit. "Heck no," she told reporters when asked.
"We are not doing it out of politics," she said of the report. "This is not the job of the IMF. We are doing it because it [Brexit] is a significant downside risk."
She said it was her "duty" to "lay out the facts and the numbers" of Brexit.
In its report, the IMF warns: "A vote for exit would precipitate a protracted period of heightened uncertainty, leading to financial market volatility and a hit to output."
Introducing Lagarde, Osborne said the IMF had made it clear that a vote to leave the European Union will "cost us money".
Lagarde also said there was "level of anxiety" around the world about Brexit. The referendum, she said, was an international issue as well as a domestic UK issue.
Lord Lamont, the former Conservative chancellor, said: "This daily avalanche of institutional propaganda is becoming frankly ludicrous. Important institutions are being politicised and used to make blood-curling forecasts."
Her warning came after Mark Carney, the governor of the Bank of England, said Brexit risks "could possibly include a technical recession".
"Not so long ago the Chancellor berated the IMF for being too pessimistic about the British economy. Now he cheers them on.
"Christine Lagarde says what happens in Britain affects other countries in Europe including France. Perhaps what she is really afraid of is that if Britain leaves the EU other countries in the EU will want to hold referendums on their membership of the EU."
Tory employment minister Priti Patel said the IMF was guilty of "talking down Britain".
"The EU-funded IMF should not interfere in our democratic debate a week before polling day. It appears the Chancellor is cashing in favours to Ms Lagarde in order to encourage the IMF to bully the British people – it is a sign of the desperation in the IN campaign," she said.
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