Disposable incomes of middle earners have fallen by about £1,200 over the past few years, according to new research.
The Office for National Statistics said that between 2007/8 and 2010/11, the average disposable income for non-retired households in the middle fifth of the income distribution fell by 3.8% from £31,000 a year to £29,900.
The average fall was bigger before taxes and cash benefits were taken into account - dropping from £36,400 to £33,200.
TUC general secretary Frances O'Grady commented: "These figures show the real cost of the UK's living standards crisis for middle Britain, with households thousands of pounds poorer today than five years ago.
"But rather than helping families, the government's real-terms cut in welfare support next month will make them ever poorer."