Some families could lose around 20 times more than they gain from tax changes by April next year, the TUC has warned.
The union umbrella group 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.
The TUC has updated its tax credit calculator to allow families to work out roughly how they are affected by changes taking place from this Friday, the start of the new tax year.
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 on Friday.
"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 childcare costs and couples working from 16 and 24 hours a week between them are most affected."
The TUC said that a family with a stay-at-home mother and a father working part-time on £19,000, a 12-month-old child and a four-year-old will gain £191 from changes in the personal allowance by next April, but they will lose around £4,500, or 24 times as much, from tax credit changes.
A two-earner family with a combined income of £40,000, with two children and childcare costs of £300 a week, will gain the maximum possible from the personal allowance increase at £381 but they will lose six times as much (£2,312) from tax credit changes, it said.
TUC general secretary Brendan Barber said: "Millions of people will be getting a small boost from the personal allowance increase this Friday, but working families are likely to have lost far more from cuts to tax credits."
At present, households earning up to £41,300 a year tend to be eligible for child tax credits but the limit will go down for most people from Friday.
The TUC's tax credit calculator can be found through the TUC Touchstone blog and the Mumsnet website.