More than three million households could see their incomes fall as a result of George Osborne's tax credit cuts, the Government has been warned.
Trade union Unison claimed that a fifth of households, including up to 2.7 million with children, would be hit by the "cruel" cut which could see them face "going under".
General secretary Dave Prentis called for ministers to reveal the number of households which will lose out from the controversial changes, which have seen the Chancellor come under pressure to find a way to soften their impact on the working poor.
Ministers have said that despite the £4.4 billion tax credit cut, 80% of households will be better off by 2017-18 as a result of the tax and benefit changes introduced by Mr Osborne, including the introduction of the National Living Wage.
But Mr Prentis said the Chancellor should admit how many of those on tax credits will be worse off as a result of the measures, which will be introduced in April.
The union said if four-fifths of the 17.4 million working households will be better off by 2017/18, one fifth - 3.5 million - will suffer.
That figure would include the "vast majority" of the 3.3 million low to middle-income working households on tax credits, 2.7 million of which have children, Unison claimed.
Mr Prentis said: "Ministers are on the back foot over tax credits. They know they are and that's why they keep hiding behind misleading numbers. It's high time they came clean over the true number of tax credit losers.
"MPs are fortunate, they've just had a 10%, but cuts to tax credits could see some hard-pressed working families lose as much as 10% of their income.
"Tax credits are not a luxury that struggling working parents can do without. They are quite simply the difference between families keeping their heads above water and going under.
"Some working households will be down as much as £50 per week, a loss that will play havoc with their finances.
"The Government's cruel tax credits cuts will snatch billions from the pockets of mums and dads who are going out to work to give their children the best start in life they can.
"Several million low to middle income working households are finding it hard to understand why the Government seems so determined to punish them in this heartless way."