Chancellor George Osborne, a Conservative, has put party politics aside joined forces with Labour's former shadow chancellor Ed Balls and former Lib Dem business secretary Sir Vince Cable to urge people to vote to stay in the EU.
Making a speech at Stansted airport, Osborne accused the Leave camp in the EU referendum of indulging in conspiracy theories as he insisted there was an "overwhelming consensus" among economists and world leaders that Brexit would be bad for the UK.
He added 450 jobs and almost £1 billion in investment announced by Ryanair would be "at risk if we left the EU".
The Chancellor's speech followed more than 300 business leaders signing a letter urging Britain to vote to leave the EU in the June 23 referendum.
In a letter to The Daily Telegraph, they argued that businesses will be "free to grow faster, expand into new markets and create more jobs" if they are unconstrained by EU rules.
But Osborne said new Treasury analysis showed if the UK was 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 £200 billion within 15 years.
According to the Chancellor, observers ranging from the Bank of England and the International Monetary Fund to the OECD and US president Barack Obama have reached a consensus in recent days that "Britain will be poorer and British people will be poorer" if the UK votes to leave the EU.
He also accused the Leave camp of responding by treating the warnings as "a massive conspiracy".
"It's not a conspiracy," Osborne added. "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."
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.
At Stanstead, Balls explained why he was prepared to put politics aside and share a platform with his former rival Osborne, insisting he wanted to "do his bit" in the national interest.
He said: "There are times to put party politics aside, this is one of those moments. If we, as a country, make the wrong decision in a few weeks' time and we end up - on the Treasury analysis - being poorer and excluded for years to come, that will hit families really hard, it will hit businesses really hard."