Chancellor George Osborne and his Labour predecessor Alistair Darling have joined forces to accuse leave-the-EU campaigners of "making it up as you go along".
The political opposites, united in their support for the European Union, have published an open letter calling on the Brexit campaign to answer basic questions about its economic argument.
They said the campaign was putting forward "uncosted and unworkable proposals" that would damage the country.
It comes a day after leading Vote Leave campaigners Boris Johnson and Michael Gove sparked further Tory division by suggesting the UK could adopt an Australian-style points-based immigration system in the wake of a Brexit.
The letter, addressed to Vote Leave, said: "It is simply not good enough to pretend to the British people that they can vote leave and there would not be profound and negative economic consequences that would affect them and their families.
"To avoid suspicion that you have no economic plan and are just making it up as you go along, please can you answer these basic economic questions, that the British people have still not had answers to."
The questions centred on what kind of trading relationship the UK would have with the EU, what guarantees they have that no tariffs would be imposed and how long new negotiations would take.
It also asked whether they expected job losses in the wake of a Brexit and how it would cope with a drop in net finances, predicted by the Institute of Fiscal Studies.
The pair added: "If you can't answer these straightforward questions you are confirming that a vote to leave is a leap into the dark.
"The overwhelming evidence now shows that Britain will be poorer and families of Britain will be poorer if we leave the EU. So with only three weeks to go our final question is this: when will you level with the British people about the damaging consequences of your dangerous plans?"
It comes as David Cameron prepares for his first major television interview of the EU referendum campaign.
The Prime Minister will face questions from journalists and members of the public in a live appearance on Sky News this evening.
Mr Cameron has avoided appearing alongside fellow Tories campaigning against him, with the broadcaster set to quiz Justice Secretary and leading Vote Leave campaigner Mr Gove separately on Friday night.
Meanwhile Labour leader Jeremy Corbyn is due to use a speech to claim the only threat the UK faces is from the Tories and not Brussels.
Jeremy Corbyn, who has refused to share a platform with Conservative campaigners, will argue that workers' rights have been bolstered by EU membership.
With three weeks left until voters go to the polls, Tory leave campaigners are continuing their attack on the European Union with ex-defence secretary Liam Fox claiming a Brexit would help ease competition in the housing market by cutting immigration.