Footfall fell 2.6% in June, the fifth monthly drop this year, with out-of-town retail destinations showing the biggest decline at 5.8%.
This compares to a 1.5% fall in those visiting town centres, according to the Experian National Retail Footfall Index.
Experian's director of retail consultancy Jonathan de Mello said: "The reduction in visits to out-of-town centres is having a significant impact on retail sales because these visits have a much better conversion to sale ratio and generate much higher value purchases for retailers. People generally drive to out-of-town destinations with a specific large purchase in mind and there is far less browsing than is the case in town centres."
Experian senior economist Matthew Sherwood added: "Consumer confidence is at an 18-year low and this is having a significant impact on consumer spending. The squeeze on incomes from rising food and fuel prices continues to intensify, house prices are falling and there are no signs that the labour market is beginning to turn."
Sherwood forecasted a further slowdown in retail activity for July.
Retail business failures are also on the rise, according to Experian's latest insolvency figures. There have been 282 non-food retail business failures in the last three months, up almost 25% on the 226 failures reported for the same periodlast year.
Separately, McArthurGlen Designer Outlets in the UK has defied the downward trend, by posting a sales jump of 3% in the six months to June. Sales at its premium and designer stores, which include Burberry, Calvin Klein and Lacoste, saw an uplift of 19%, with the average spend per customer increasing by more than 7%.