Despite this decline, total footfall in the country improved by 1.8 percentage points from July, but was worse than the three-month average decrease of 12.3%.
On a regional basis, England recorded an 11.2% decline in footfall, while Northern Ireland and Wales reported declines of 11.5% and 13.1% respectively.
Scotland reported the greatest fall in footfall with 14.8%, but saw a slight improvement from July.
BRC chief executive Helen Dickinson said: “Whilst footfall in August continued its modest recovery towards pre-pandemic levels, the rate of improvement slowed.
“Many people remain concerned about the rising cost of living and the price of their energy bills, which has kept them away from visiting high streets and town centres.
“With consumer confidence at historic lows, stores continue to focus on converting customer footfall into retail sales.
“Big events in Birmingham and Edinburgh saw more notable advancements to footfall, as the Commonwealth Games and the Edinburgh Fringe brought more shoppers in.
“September brings a new Prime Minister and new government with a difficult task ahead.
“To help retailers keep prices as low as possible, the government should include a freeze in the business rates multiplier next year on the to do list, otherwise the 10% inflationary increase in rates bills will lead to higher prices for customers.”
Sales volumes were higher by 2.3% compared with pre-coronavirus (Covid-19) levels in February 2020, but were still down over the past year.