Freezing child benefit will cost families with two children more than £400 over the next few years, according to a new study.
The TUC said the Government's decision would lead to a real-terms cut in child benefit of £57.20 this year, £153.40 in 2012/13 and almost £200 during 2013/14.
General secretary Brendan Barber said: "Freezing the value of child benefit has already hit millions of families hard, and by 2014 many will be over £400 worse off.
"By ploughing ahead with the introduction of a costly means-test, the Government is only making the problem worse - introducing confusion and uncertainty into a system that used to work well as a way to provide vital support to millions of families.
"The best way to make the rich pay their share is through the tax system, but this Government has done the opposite by reducing taxes for the very wealthiest.
"Cutting the value of benefits for children and introducing complicated new assessments means those in greatest need will get less, with the poorest children suffering the most."