The campaign to keep Britain in the European Union received a major boost yesterday after the G20 group of the world’s most powerful countries warned that a Brexit vote would cause a “shock to the global economy”.

George Osborne, in China for the meeting of global finance ministers, said the warning exposed the danger to living standards and the economic recovery.

He also launched a thinly veiled attack on the London Mayor Boris Johnson over his decision to back a leave vote – insisting that the decision was not “some amusing adventure into the unknown”.

Mr Johnson has taken a back seat since revealing he would campaign to leave the EU last week. The Independent on Sunday understands he is being urged not to back the Eurosceptic “Vote Leave” campaign amid growing Tory fears the rival Ukip-backed group “Grassroots Out” has taken pole position in the race to be designated the official Brexit body by the Electoral Commission.

Close allies of the London Mayor have warned him in private that he should not allow himself to become the “plaything of rival campaigns” who are locked in a fierce and increasingly bitter campaign to win official designation in order to receive public funds for the referendum campaign.

All six cabinet ministers who publicly back Brexit have joined the Vote Leave campaign run by Michael Gove’s divisive former aide Dominic Cummings. But Mr Johnson has kept his distance.

The London Mayor’s position was, however, lambasted by Mr Osborne. “A British exit would hurt people’s jobs, livelihoods and living standards – it’s deadly serious,” he said. “It’s my responsibility as Chancellor to make it clear to people what the economic risks are – and that we are stronger, safer and better off remaining in a reformed EU.”

The risk of a British exit from the European Union was included in the G20’s final communique released by finance ministers in Shanghai, China on 27 February.

The Italian Finance Minister, Pier Carlo Padoan, said that a decision by Britain to leave the EU would be classified as “a powerful geopolitical shock”.

Grassroots Out at Ukip’s conference in Llandudno on Saturday (AFP)

Mr Osborne welcomed the public warning: “Finance leaders and central bank governors of the world’s biggest economies have raised serious concerns about the risks posed by a UK exit from the EU. They have concluded unanimously today that what they call the shock of a potential UK vote to leave is among the biggest economic dangers this year.

“If that’s their assessment of the impact on the world economy, imagine what it would do to the UK.”

However, the former Conservative chancellor Lord Lawson ridiculed the G20 warning. The Vote Leave chairman said: “The notion that the UK leaving the EU would cause an economic shock is absurd. Fifteen of the members of the G20 are outside the EU, and that hasn’t caused an economic shock. Indeed, most of them are doing better than most of the members of the European Union.”

A source close to Mr Johnson insisted that he respected Mr Cameron and Mr Osborne – but just thought they were “wrong”. The source added that Mr Johnson would “throw himself into the campaign” for a vote to leave the EU but “no decision had been made” yet over what role he would take.

The London Mayor met the Justice Secretary, Michael Gove – a key member of the Vote Leave campaign – last week and will hold talks with him again in the next few days.

Sources in Vote Leave said they were not worried about the Mayor’s loyalties, and insisted they were confident of being given the designation as the “official” leave campaign.

What has the EU ever done for us? Show all 7 1 /7 What has the EU ever done for us? What has the EU ever done for us? 1. It gives you freedom to live, work and retire anywhere in Europe As a member of the EU, UK citizens benefit from freedom of movement across the continent. Considered one of the so-called four pillars of the European Union, this freedom allows all EU citizens to live, work and travel in other member states. What has the EU ever done for us? 2. It sustains millions of jobs A report by the Centre for Economics and Business Research, released in October 2015, suggested 3.1 million British jobs were linked to the UK’s exports to the EU. What has the EU ever done for us? 3. Your holiday is much easier - and safer Freedom to travel is one of the most exercised benefits of EU membership, with Britons having made 31 million visits to the EU in 2014 alone. But a lot of the benefits of being an EU citizen are either taken for granted or go unnoticed. What has the EU ever done for us? 4. It means you're less likely to get ripped off Consumer protection is a key benefit of the EU’s single market, and ensures members of the British public receive equal consumer rights when shopping anywhere in Europe. What has the EU ever done for us? 5. It offers greater protection from terrorists, paedophiles, people traffickers and cyber-crime Another example of a lesser-known advantage of EU membership is the benefit of cross-country coordination and cooperation in the fight against crime. What has the EU ever done for us? 6. Our businesses depend on it According to 71% of all members of the Confederation of British Influence (CBI), and 67 per cent of small and medium-sized enterprises (SMEs), the EU has had an overall positive impact on their business. What has the EU ever done for us? 7. We have greater influence Robin Niblett, Director of think-tank Chatham House, stated in a report published last year: “For a mid-sized country like the UK, which will never again be economically dominant either globally or regionally, and whose diplomatic and military resources are declining in relative terms, being a major player in a strong regional institution can offer a critical lever for international influence.

A spokesman said: “We are confident that we will get the designation because of our cross-party nature and the professional campaigning requirement. We wish all groups well.”

But the group’s cross-party support is currently limited to the former Labour foreign secretary Lord Owen – now a crossbench peer –and Labour MP Graham Stringer.

The Government will set out the specific criteria for deciding how the Electoral Commission should decide which group is given official campaign status in Parliament tomorrow.

The rival out campaigns – Vote Leave and Grassroots Out – will have four weeks to apply for official designation. The Electoral Commission will then have two weeks to decide before announcing its choice on 14 April. Tory MPs believe this could give the Government a crucial six-week head start on the leave campaign because of the infighting between the rival groups.

But they are also increasingly concerned that the Vote Leave campaign is too narrowly Conservative to win official status.

Grassroots Out is backed by Ukip’s Nigel Farage, Respect’s George Galloway and Labour’s Kate Hoey. It has also been supported by Tory MPs Peter Bone and Tom Pursglove.

A source close to Mr Johnson said: “The campaign designation isn’t until April 14, which means the Government can just plough ahead, promoting its own policy.

“Boris must not become the plaything of rival campaigns. He would be wise to keep clear of the big two campaigns. They are clearly riven by ego.”