Pressure on George Osborne to publish details of the "true picture" of the impact of tax credit cuts on low-wage families has been joined by an influential cross-party panel.
Frank Field, who chairs the work and pensions select committee, has written to the Chancellor complaining that MPs had been left in the dark about the effects of the controversial welfare squeeze.
Plans to slash £4.4 billion off welfare spending by cutting the payments were thrown into turmoil when the Government suffered a double defeat in the House of Lords - an a rebellion by 20 Tories in the Commons.
The Chancellor has promised to draw up a set of revised proposals to ease the transition to the new system for those affected, which he will set out in next month's Autumn Statement.
But he has repeatedly declined to reveal Treasury calculations about the overall effect of the changes - which the Government claims are offset by other reforms such as a higher minimum wage.
The committee has asked for a distributional analysis of the combined impact of the changes to tax credits, the minimum wage, higher income tax personal allowance and childcare on the 3.2 million families who will be affected.
Mr Field, the former Labour social security minister who led the Commons debate, said: "The high level of cross-party concern on this issue has become very clear over the past couple of weeks.
"Yet we still have no clear picture of what the net effect of these combined changes will be on the 3.2 million families, many on low incomes, who will be affected.
"It is impossible for members to represent the interests of their constituents without this information, and we call on the Government to provide the true picture urgently."