UK families were £15 a week worse off in September according to the Asda Income Tracker, representing the biggest drop since records began.
Households had on average £163 of discretionary income a week in September, down 8.4% a year earlier.
It is the third consecutive month of record-breaking decline.
More than a third of respondents said the cost of utilities is the “biggest drain on the family purse”. Inflation growth also put pressure on household spending power, as well as the rising cost of transport.
Asda president and chief executive Andy Clarke said: “For 18 consecutive months we’ve seen a decline in family spending power. While disposable income was down everywhere in September, there is clearly a growing divide between the North and the South.
“Spiralling petrol costs are piling on extra pressure on households across the north of England and Northern Ireland where families are much more reliant on the car to get about.
“As we head into winter and the nights draw in we know that the cost of food, transport and utilities go up. It’s our job to continue to cut costs across all our operations so that we can hold down the price of food and fuel and help hard-pressed families make ends meets as prices are rising everywhere else.”
Charles Davis managing economist at the Centre for Economics and Business Research, which compiles the study, said: “Family spending power continues to be under significant pressure from the three-pronged threat of fragile wage growth, rising unemployment levels, and soaring consumer price inflation.
“As utility price increases continue to feed through into erosions in family budgets, a tough couple of months lie ahead. However some pressure could ease from the start of 2012, as inflation is expected to fall back.”