Asda said that although average earnings before tax had gone up £24 a week over the last year, the average family was £1 a week worse off because tax, food, housing and transport cost £25 a week more.
Asda said the average family had 1% less disposible income in March 2008 compared with March 2007. The cost of living has continued to rise by 5.2% while net income was up just 3.6%.
The price of clothing was down 5.2% but essentials such as food and transport rose 5.5% and 7.1% respecitvely, squeezing shoppers' spending.
Asda chief executive Andy Bond said: "March's data reveals that the growth in household inflation has actually levelled out at 5.2%. However, as retailers we must redouble our efforts on behalf of our customers, to not pass on any unnecessary price rises. In practise, that means challenging both ourselves and our suppliers to lower costs even further, to keep a lock down on inflation".
Bond added: "The latest figures show that despite a moderate growth in income, families are still £1 a week worse off compared to this time last year. This highlights the importance of having a competitive retail sector, that drives down prices and enables to economy to grow."
Asda's Income Tracker research s compiled by the centre for economics and business research.