David Cameron will join forces with leading figures from the left to accuse members of his own Cabinet of conning the British public.
The prime minister, backed by Labour’s Harriet Harman, Liberal Democrat Tim Farron and the Greens’ Natalie Bennett, will ramp up the pressure on the Leave camp by calling for it to spell out an economic plan for the UK if it quits the bloc, the Press Association reported.
But Boris Johnson will warn Britain faces a "triple whammy of woe" that includes footing an extra £2.4 billion bill from Brussels if it remains part of the European Union.
Hikes in the EU budget and the threat of more euro bailouts mean the risks of remain are "massive", the former London mayor will claim.
Both camps will hold high-profile events after a weekend of heightened party divisions as the tensions of the referendum campaign took their toll.
Former prime minister Sir John Major used a television interview to brand Johnson a "court jester" and accuse the Leave camp of being "deceitful".
Senior Tory Jacob Rees-Mogg said the attack was "hypocritical and untrue" and was fuelled by the ex-premier’s "bitterness" over his own failings on Europe.
"What we have heard today are the bitter ramblings of a vengeful man," he told BBC Radio 4’s Westminster Hour.
Lord Prescott said Labour was "absolutely" failing to galvanise its supporters and admitted leader Jeremy Corbyn was "not a passionate man".
Cameron, Harman, Farron and Bennett, will unveil a dossier on the "often contradictory" statements the Leave campaign has made on the economy and claim it has had 23 positions on the alternative to the single market.
Refusing to set out an economic plan for Brexit is "reckless and undemocratic" because it denies people the chance to assess what they are voting on, they will argue.
"It’s time for the Leave campaign to outline their economic plan for Britain outside Europe," they will say.
“They are perpetuating an economic con-trick on the British people, and we’re calling time on it.
"The British public deserve better than being asked to roll the dice."
Vote Leave claimed a review of the EU’s multi-annual financial framework has been delayed until after the June 23 referendum. It says a £19.4 billion backlog in unpaid bills and other hidden costs means the UK is set to be hit with a £2.4 billion bill.
The EU’s budget is also set to increase to deal with the impact of migration, according to the Brexit camp.
And Britain can still be forced to bail out other EU countries, it insisted.
Johnson said: "The risks of remain are massive. Not only do we hand over more than £350 million a week to the EU, but if we vote to stay the British people will be on the hook for even more cash.
"It is a triple whammy of woe: the eurozone is being strangled by stagnation, unemployment and a lack of growth, it could explode at any time and we will be forced to bail it out. The botched bureaucratic response to the migration crisis means the Eurocrats are demanding even more of our money.
"And now we find that there is a £20 billion black hole in the EU’s finances.
"If we vote to stay in the EU we will be forced to hand over even more money to Brussels. If we Vote Leave we can avoid this extra £2 billion bill from Brussels, take back control of our money and spend it on our priorities like the NHS."
Labour’s Chuka Umunna said: "This is nonsense. Our special status in Europe means we are protected from paying into eurozone bailouts, we have already cut the EU budget and we have a veto over it in future."
Vote Leave chief executive Matthew Elliott said Cameron’s attack was a sign of desperation.
He said: "This is desperate stuff from an increasingly desperate campaign. Number 10 are panicking and are resorting to petty personal attacks because they know they are losing the arguments. People are rejecting their pessimistic campaign of doom in favour of our positive vision for Britain’s future."
Britain’s biggest trade unions, meanwhile, have warned their six million members that quitting the EU would allow a Tory government to dismantle worker rights on parenting, holidays and equality.
In a letter to the Guardian, union leaders, including the general secretaries of Unite, Unison, the GMB and Usdaw, warned the Conservatives would "negotiate away our rights" if voters backed Brexit.
"Throughout the 1980s and early 1990s the British trade union movement worked in solidarity with our European partners and fought hard to secure valuable working rights legislation at EU level," they said.
"To this day these rights - including maternity and paternity rights, equal treatment for full time, part-time and agency workers and the right to paid leave - continue to underpin and protect working rights for British people.
"If Britain leaves the EU we are in no doubt these protections would be under great threat. Despite words to the contrary from figures like Iain Duncan Smith and Michael Gove, the Tories would negotiate our exit and, we believe, would negotiate away our rights.
"We simply do not trust this government if they are presented with an unrestricted, unchecked opportunity to attack our current working rights."
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