The British Retail Consortium (BRC) said the value of total sales was 1% lower in June than a year ago.
That followed a 1.1% decline in May.
Like-for-like sales, which adjusts for changes in floor space, fell 1.3%.
However, the figures are not adjusted for inflation, which the BRC said was running at 3.1% among its members – the highest since 2008 as the cost of living crisis gathers pace.
The official consumer prices index measure of inflation facing consumers is at a 40-year high of 9.1% – led mostly by energy and fuel costs.
Separate data from Barclaycard on Tuesday showed consumer spending in June was 6.2% higher than a year earlier, pushed by a nearly 40% jump in spending on utilities.
