The Prime Minister was joined by senior figures from opposition parties to claim that leaving the single market without a clear economic plan would have “a profound and damaging effect on our economy”.
Standing in front of a row of Minis painted in each party’s colours, Mr Cameron said he was “proud” to appear alongside Labour’s Harriet Harman, Lib Dem leader Tim Farron and Green leader Natalie Bennett in an “unprecedented” show of support for staying in the EU.
As two polls showed the Leave campaign pulling into the lead, the Prime Minister warned that Brexit would be followed by a “decade of uncertainty” as Britain attempts to negotiate new trade arrangements with the rest of the world.
“Add those things together – the shock impact, the uncertainty impact, the trade impact – and you put a bomb under our economy,” Mr Cameron said. “And the worst thing is we’d have lit the fuse ourselves.”
Ukip leader Nigel Farage dismissed Mr Cameron’s language as “hysterical”, adding: “The Prime Minister is clearly convinced he is losing the argument.”
Tory MP Boris Johnson said it was a “delusion” to think Britain could boost its prosperity by “bartering away our freedom and democracy”.
He warned of a “triple whammy” of economic woe if the UK remains in the EU, claiming that Brussels was waiting until after the referendum to seek an increase in budget contributions as well as extra cash to fill a £20 billion black hole in unpaid bills.
The four remain-backing politicians accused Leave campaign spokesmen of making unfunded spending commitments that would open up a £153.6bn black hole in public finances, along with plans to scrap workers’ rights and environmental protections.
Ms Harman accused the Leave camp of “speaking in code” about wanting to scrap EU regulations, which would mean cuts to parental leave, while Mr Farron accused Leave of running “a campaign based on lies”.
There was a further warning about the economic impact of leaving the single market from the Business Secretary, Sajid Javid, who claimed firms could face a £34.4bn “export tax” on trade with the EU.
In a joint letter to the Vote Leave campaign with former EU trade commissioner Lord Mandelson, Mr Javid said exporting firms would pay an average of £80,000 in additional costs.
“A campaign to leave the EU’s single market without a plan for an alternative is an act of economic sabotage which would risk thousands of jobs, billions of trade and investment and the future economic stability of our country,” Mr Javid and Lord Mandelson said.
Their analysis was dismissed as “scaremongering” by Tom Harris of the Scottish Vote Leave campaign, who said many nations outside the EU trade with member states.
The Institute of Fiscal Studies warned yesterday that the cost of leaving the EU could outstrip the £8bn saved on the UK’s contribution to the 28-nation bloc, undermining claims that Brexit could boost investment in the NHS.
The economic think-tank said taxes or borrowing may have to rise in order to maintain public spending levels, adding there was “virtual unanimity” among forecasters that Brexit would damage the public finances.
Former chancellor Lord Darling said: “The NHS, and everyone who relies upon it and works in it, is stronger thanks to our membership of the EU.”