Good Friday will turn into bad Friday for hundreds of thousands of families on low incomes as tax changes swing into action.
The TUC warned on Wednesday that families could lose around 20 times more than they gain from tax changes by April next year, with the Child Poverty Action Group condemning the government's decision to press ahead with the tax credit rule change as "reckless."
The TUC said families will have gained up to £381 by April 2013 due to the personal allowance threshold rising faster than inflation.
But they could lose out on more than £4,000 following changes to working tax credits over the same period.
It said: "Ministers have been keen to highlight the hundreds of pounds that millions of families - and anyone else earning up to £100,000 - will receive from the personal allowance threshold being raised to £8,105 tomorrow.
"However, the government has been less keen to highlight the far greater losses that families will have incurred from changes to working tax credits and the freezing of child benefit.
"Families with high child-care costs and couples working from 16 and 24 hours a week between them are most affected."
TUC general secretary Brendan Barber said: "With unemployment at a 17-year high and full-time jobs being replaced with part-time ones, parents struggling to find 24 hours of work between them could lose thousands of pounds."
Child Poverty Action Group spokesman Tim Nichols said: "Good Friday will feel like 'bad Friday' this year for hundreds of thousands of families on low earnings.
"The government must suspend this change and accept the reality that these families are simply unable to get additional hours of work."
HM Revenue and Customs apologised last month after it sent out about a million letters inadvertently containing potentially misleading information about child tax credits.
The letters said the maximum household income for a family to be eligible for the benefit will be £26,000, a figure which is broadly correct for those with one child but rises for those with more.
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