THE downgrade in UK growth by the International Monetary Fund for fear Britain could leave the European Union was the clearest warning of the “taste of bad things to come” if the vote on June 23 was for so-called Brexit, George Osborne has insisted.

As the chancellor and other pro-EU politicians seized on the global economic body’s intervention as supporting their argument for Britain to remain in the Brussels bloc, the Vote Leave campaign accused the IMF of “talking down the British economy…at the request of our own chancellor”.

In its World Economic Outlook, the IMF warned that Brexit could inflict "severe regional and global damage" by disrupting trade relations.

It said negotiations over post-exit arrangements would probably be "protracted", leading to an "extended" period of uncertainty and market volatility.

The Washington-based body scaled back its projection of UK economic growth for 2016 by 0.3 percentage points to 1.9 per cent - marginally below the 2.0 per cent forecast of the UK Government's independent forecaster, the Office for Budget Responsibility - but held its forecast for 2017 at 2.2 per cent.

Mr Osborne, describing the IMF remarks as a stark warning, said: "If the British economy is hit by the mere risk of leaving the EU, can you imagine the hit to people's income and jobs if we did actually leave? The IMF has given us the clearest independent warning of the taste of bad things to come if Britain leaves the EU."

David Cameron also seized on the IMF’s comments, tweeting: “It is right; leaving the EU would pose major risks for the UK economy. We are stronger, safer and better off in the European Union."

And David Miliband described the IMF intervention as an "absolutely devastating" judgement on Brexit.

In a speech in London, the former Labour foreign secretary who quit frontline politics to head the International Rescue Committee aid organisation, argued that Brexit would amount to "unilateral political disarmament" of the UK's influence in the world.

"No nation in human peacetime history, never mind Britain, has voluntarily given up as much political power as we are being invited to throw away on June 23," he said.

John McDonnell for Labour said the IMF downgrading of growth forecasts “should act as a signal that George Osborne needs to change course and that Tory backbenchers who wildly scream for Brexit should think again”.

Meantime, the Vote Leave campaign rubbished the IMF’s intervention with Matthew Elliott, its chief executive, saying: "The IMF has talked down the British economy in the past and now it is doing it again at the request of our own chancellor. It was wrong then and it is wrong now.”

Nigel Farage, the Ukip leader and Grassroots Out spokesman, said the IMF could “not be trusted" as it had been "hijacked by the architects of the failing EU project".

In the Commons, Alex Salmond claimed British voters and European citizens were crying out for an EU debate, which "rises above the internal divisions of the Conservative Party".

During Foreign Office Questions, the former SNP leader goaded the Tories, saying that when the Prime Minister said European discussions were abrasive and difficult, he was not talking about other EU member states or Commons debates or even within the Conservative Party, “he was taking about discussions within his own Cabinet”.

Momentum in the EU referendum campaign is set to move up a gear this week.

Labour leader Jeremy Corbyn will make his first speech on the EU tomorrow in London. Thursday is also the deadline for the Electoral Commission to announce which groups will become the official Remain and Leave campaigns; those designated will be able to spend up to £7 million, be entitled to campaign broadcasts and free distribution of information as well as £600,000 for administration costs.

On Friday, the official 10-week designated period of the referendum campaign begins.