Stripping out the impact of new floor space, retail sales values sank by 0.4 per cent in August from the same month a year earlier, the British Retail Consortium said in a statement on Tuesday. Non-food sales were particularly weak.
That was the lowest level since November last year when excluding April, which was heavily distorted by the timing of Easter.
"The feel good factor from the Olympics failed to inspire spending," said the BRC, which represents 60 per cent of retailers in Britain.
Total sales - including stores that opened within the past 12 months - rose 1.6 per cent over the same period. However, that marked a slowdown from growth of 2.0 per cent in July and 3.5 per cent in June.
"There's no evidence here of any Olympic boost to retail sales overall," said Stephen Robertson, director general of the BRC, whose survey covered the period from July 29 to August 29. The Olympics ended on August 12.
He added: "Hot weather and the Olympics did help sales of party food and drink but that was more than offset by a really weak performance for non-food goods.
"It's clear people were absorbed by the magnificent Olympics and had little interest in shopping, especially for major items."
The BRC also revealed that online sales grew by 4.8 per cent in August. But that was the lowest level since it began collecting internet sales data in 2008.
"Some retailers told us online activity was particularly thin in the evenings. If people weren't watching television they were more likely to be following the sport on PCs and mobile devices than shopping," added Robertson.
"As summer gives way to the all-important Christmas run-up, retailers will be hoping sales that didn't happen in August have been postponed and not lost entirely."
Economists added that retail sales may have also been hampered by people avoiding the shops in London during the Olympic Games because of fears of excessive congestion on the transport system.