LEAVING the European Union would be a “one-way ticket to a poorer Britain,” George Osborne has declared as he mockingly accused the Leave camp of indulging in conspiracy theories.

The Chancellor was joined by former adversaries Ed Balls and Sir Vince Cable for a pro-EU campaign even at Stansted airport, stressing how there was an "overwhelming consensus" among economists and world leaders that Brexit would be bad for the UK.

He claimed that 450 jobs and almost £1 billion in investment recently announced by Ryanair would be "at risk if we left the EU".

Mr Osborne spoke as more than 300 business leaders signed a letter urging Britain to vote to leave the EU in the June 23 referendum, arguing that the UK's competitiveness is being undermined by its membership.

In a letter to The Daily Telegraph, they argued that businesses would be "free to grow faster, expand into new markets and create more jobs" if they were unconstrained by EU rules.

But the Chancellor stressed how new Treasury analysis showed that if the UK were forced to rely on World Trade Organisation rules following Brexit, it could expect to lose trade worth £200 billion a year and overseas investment worth £200bn within 15 years.

Recent days had seen the emergence of a consensus shared by observers ranging from the Bank of England and the International Monetary Fund to the OECD and US president Barack Obama that "Britain will be poorer and British people will be poorer" if the UK votes to leave the EU, said the Chancellor.

And he accused the Leave camp of responding by treating the warnings as "a massive conspiracy".

"That's everyone from Mark Carney to Christine Lagarde to Barack Obama to the entire editorial team at ITV to the staff at the IMF and OECD, to hundreds of economists, to a majority of leaders of small, medium and large firms - they think they are all part of some global stitch-up to give misinformation to the British people," said Mr Osborne.

"The next thing we know, the Leave camp will be accusing us of faking the moon landings, kidnapping Shergar and covering up the existence of the Loch Ness monster.

"The response to the sober economic warnings from around the world by those who want to leave the EU has not been credible or serious."

Mr Osborne said that it was "difficult to think of more credible observers" of the UK economy than the IMF and the Monetary Policy Committee of the Bank of England, who had each given a clear message that "a vote to leave could see family incomes fall, growth hit and borrowing costs rise".

"They join a line of observers that range from the OECD to the London School of Economics to the eight former US treasury secretaries to the president of the United States of America, to the prime minister of Japan, to the leaders of Australia and New Zealand," said the Chancellor.

"Indeed every member of the G20, every one of our major trading partners and every major international financial institution has been unequivocal that leaving the EU would come at an economic cost."

He added: "It's not a conspiracy. It's called a consensus. The interventions of the last couple of weeks, from the IMF to the Bank of England, make very clear that the economic argument is beyond doubt - Britain would be worse off if we leave the EU, British families will be worse off, equivalent to £4,300 a household.

"Leaving the EU is a one-way ticket to a poorer Britain."

Mr Osborne claimed that leading advocates of Brexit wanted not only to leave the EU, but also to take Britain out of the single market.

He said this would leave the UK reliant on WTO rules to govern its trade with the rest of the world, which would be "a disaster for the British economy".

"New Treasury analysis shows that if we left the single market and relied on the rules of the WTO, then after 15 years we would be doing £200 billion less trade every year, in today's terms, and we would miss out on over £200 billion of overseas investment into our country," said Mr Osborne.

"It means we don't see the new jobs and facilities like we see here today at Ryanair. It means less investment in offices, factories, car plants, high street shops, local industrial estates.

"What does all this mean for you? It means fewer jobs, lower incomes and higher prices in jobs."

Signatories to the letter include Peter Goldstein, a founder of Superdrug; Steve Dowdle, a former vice-president Europe of Sony; David Sismey, a managing director of Goldman Sachs, and Sir Patrick Sheehy, the former chairman of British American Tobacco.

"Brussels's red tape stifles every one of Britain's 5.4 million businesses, even though only a small minority actually trade with the EU," they said.

"It is business - not government - which generates wealth for the Treasury and jobs for our communities."

Boris Johnson, who is resuming a Vote Leave bus tour of the UK, said EU membership was favoured by "fat cat" bosses who exploited cheaper labour and took advantage of "remote and opaque" regulations to stifle competition and increase their own pay.

Writing in the Daily Telegraph, he said: "They are getting personally richer and richer, while those at the bottom have seen a real terms fall in their wages; and they are getting richer by mainlining immigrant labour for their firms and manipulating EU regulation that only the big players can understand.

"If you want to back the entrepreneurs, the grafters, the workers, the innovators, the burgeoning and dynamic businesses of Britain - then Vote Leave on June 23, and give this cabal the kick in the pants they deserve."

Meanwhile, David Cameron has warned that leaving the EU would be a "national error" as part of an attempt to reach out to Labour supporters.

In an article for the Labour-supporting Daily Mirror, the Prime Minister said he was backing Remain for the sake of jobs, security and Britain's place in the world.

His intervention came amid concerns in the Remain camp that they are encountering significant hostility to the EU in traditional Labour areas.

On Saturday, Jeremy Corbyn appealed to supporters at a Labour In rally in London to blame the Conservatives, not Brussels, for the problems facing the country.

In his article, Mr Cameron emphasised he had been campaigning alongside Labour former foreign secretary David Miliband and ex-TUC general secretary Sir Brendan Barber, and praised Gordon Brown's "powerful and passionate" speech in support of Remain.