David Cameron: Brexit would 'put a bomb under our economy'

Both the SNP’s Nicola Sturgeon and Labour’s Jeremy Corbyn have made clear they will not share a platform with the Conservative leader.

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

Read more: John Major accuses Leave camp of deceit as he brands Boris Johnson a "court jester"

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."

Read more: David Torrance: Why SNP's support for European Unionism jars with its rejection of UK Unionism

He then added that the shock impact, the uncertainty impact, the trade impact of Brexit meant “you put a bomb under our economy. And the worst thing is we’d have lit the fuse ourselves”.

Harriet Harman, Labour’s former deputy leader, said it was not surprising Labour supporters had struggled to catch a glimpse of why Labour backed the EU given the media had been dominated by the blue on blue attacks within the Tory party.

Emphasising the need to stay in the EU to protect the rights of British workers, she said: “Because we’re in the EU, people have better rights at work. The EU guarantees those rights. It’s the EU that made our governments pass laws to ensure employers give paid holiday, paid maternity leave, rights for part-timers. So long as we’re in the EU no Tory government can try and take those rights away.”

Tim Farron, the Liberal Democrat leader, made clear he wanted to put a positive case for the EU but said also that it was important to expose the Vote Leave case was based on lies.

“I believe in the positive case for Europe but I cannot stand back and allow the Leave campaign to guide us towards economic ruin because of a campaign based on lies. How betrayed will people feel if they vote to leave Europe based on the reasons presented by the Leave campaign, only to see in the weeks, months and years that follow that those reasons were utter, invented rubbish,” he declared.

Earlier, the pound slumped to fresh lows in morning trading after two polls showed rising support among voters for leaving the European Union.

Sterling hit a three-week low against the dollar, falling as much as 0.9 per cent to $1.43. A YouGov poll showed that 45 per cent of voters now favour Brexit with 41 per cent opting to remain. Brexiters were also ahead in an Observer/Opinium poll by 43 points to 40.

Craig Erlam, senior market analyst at Oanda, said more volatility should be expected over the coming weeks.

"The polls are likely to make people rather uneasy and we can see that quite clearly today in the pound, currently down almost one per cent against the dollar, having hit three-week lows earlier in the session.

"With both sides likely to step up their game over the next couple of weeks, I imagine we'll see a lot more volatility in the pound, and the closer the polls get, or if "Vote Leave" continues to push ahead, the pound may find itself back towards April's lows before too long."

Several experts, including Bank of England governor Mark Carney, have warned that the value of sterling could plummet in the event of a Leave vote.

The economy had taken centre-stage in the debate until recently, with organisations such as the OECD, IMF and a plethora of economists and world leaders all warning that Brexit could batter the British economy.

However, polls suggest the Leave campaign's tactic of ramping up rhetoric on immigration is proving successful.

Pollsters will be working overtime in the coming weeks as newspapers, broadcasters and private institutions commission more surveys. However, experts believe this could lead to more volatility for the pound.

Hussein Sayed, FXTM chief market strategist, said: "It is becoming extremely worrying for the financial markets and we expect more sterling losses if polls continued to indicate a Brexit lead.

"From a technical perspective GBP/USD imminent support stands at 1.4330; however, don't expect the pair to respect any technical or fundamental indicators as polls will continue to dictate the move until June 23," he added.