Retail sales last month were down 2.1 percent on a like-for-like basis from September last year, when they increased 0.7 percent on 2012 levels.
"The prolonged Indian summer wilted retail sales in September, leaving clothing retailers hot under the collar. Selling woolly jumpers in warm weather is a tough ask, even for the most talented of sales staff," David McCorquodale, head of retail at KPMG, said in the report.
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Grocers also had a challenging month, with many announcing further price cuts. Earlier data from the BRC showed food inflation hit an all-time low in September, as supermarkets tried to attract customers with special offers.Fresh food prices remained flat in September, for the first time since February 2010.
However, retailers are on a "firm footing" as Christmas nears, according to McCorquodale.
"One warm September doesn't ruin a Christmas. The winners will be those who have invested in their systems and carefully managed their stock levels to give themselves the best shot at a successful Christmas," he said.
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