Latest EU referendum poll of polls: Brexit in front as Cameron joins cross-party Remainers

The Prime Minister appeared alongside leading figures from Labour, the Liberal Democrats and the Greens to issue a challenge to the Brexit camp to "come clean" on how they see Britain's economic future if the UK votes to leave the EU in the June 23 referendum.

The demand came as prominent Leave campaigner Boris Johnson warned Britain faces a "triple whammy of woe" if it votes to remain, including hikes in its annual contributions to EU budgets and an extra £2.4 billion bill to pay for Brussels overspending.

The cross-party Britain Stronger in Europe group released a dossier detailing 23 alternative trade models which had been espoused at various times by Leave campaigners, each of which they said would have "a profound and damaging effect on our economy".

Speaking in south London, Mr Cameron said: "We lay down a challenge today. It is time for Vote Leave to come clean about their economic plan for Britain outside Europe.

"By refusing to set out their vision, they are being undemocratic and reckless. Undemocratic because it's the duty of anyone seeking votes to tell you what they are asking you to vote for, especially when it's on something as important as our economy. And reckless because by failing to set out a concrete economic plan, they are playing with people's jobs and with people's livelihoods.

"While they peddle fantasy politics, in the real world our economy is slowing because of the huge uncertainty hanging over Britain's economic future. It is therefore unacceptable for them to continue to dodge questions.

"They are performing an economic con-trick on the British people and we are calling time on it."

Mr Cameron said he was "proud" to stand alongside Labour's Harriet Harman, Lib Dem leader Tim Farron and Green leader Natalie Bennett in "an unprecedented show of cross-party unity" in support of continued UK membership of the EU.

He warned that Brexit would put "a bomb under our economy", with recession in the immediate aftermath of EU withdrawal, a hit to UK exports and a "decade of uncertainty" as Britain attempts to negotiate new trade arrangements.

The June 23 vote represented a choice between "an economy built on solid ground or one that's like a sinkhole, crumbling beneath us; moving our economy forwards or setting it back with a roll of the dice - risking all our futures", he said.

The new dossier accused the Leave campaign of making unfunded spending commitments that would create a £153.6 billion black hole in the public finances, along with plans to scrap workers' rights and environmental protections.

Mr Cameron said advocates of Brexit had failed to explain what the UK's trade relationship with the rest of the world would be after EU withdrawal.

"They've lurched from one idea to another," he said.

"First, they said we should be like Norway, then Canada, then Albania, then America. And along the way, they've taken us from Iceland to the Isle of Man and Morocco and Moldova.

"They are sticking pins on a map. Scores of suggestions, but they add up to zero concrete plans. All we do know is that we wouldn't get as good access as we have now."

Speaking on a campaign visit to a factory in Stratford-upon-Avon, Mr Johnson said Britain was already facing a £2.4 billion bill to help cover a £20 billion "black hole" in the EU budget and would be unable to avoid further demands from Brussels.

"When you look at the deficits in the southern regions of the EU, when you look at the budgetary problems they have got, there is no doubt in my mind, as they go forward trying to keep this thing together, they are going to be calling on all EU members to try to do that and they have solid treaty basis upon which to do so," he said.