Warm weather last month delayed early sales of Christmas food such as puddings and seasonal biscuits, research suggests.
Retail analysts Kantar said the sunny weather meant fewer people had started to stock up for the festive season.
Instead, sales of ice cream, burgers and dips jumped as people continued to enjoy the sun and fire up barbecues.
A separate survey also said that the warmer weather had put off people from buying autumn clothing.
The latest retail sales monitor from the British Retail Consortium (BRC) and KPMG said sales growth slowed in September, as people limited spending to focus on household bills.
The value of sales was up 2.7% last month from a year earlier, compared to a 12-month average of 4.2%.
Paul Martin, UK head of retail at KPMG, said retail sales were continuing to "limp along".
While sales of food, drink and health and beauty products remained strong, some categories such as clothing were hit "as the unseasonal warm weather delayed trips to the shops to stock up on winter wardrobe purchases".
The data from Kantar also indicated the pace of food price rises was continuing to slow, with grocery inflation - the rate at which prices rise - down for the seventh month in a row to 11%.
Tom Steel, strategic insight director at Kantar, said that for the first time since last year, "the prices of some staple foods are now dropping".
"Dairy was one of the categories where costs really shot up last autumn but the average price paid for a 250g pack of butter is now 16 pence less than 12 months ago."
However, last month's record high temperatures for September led to changes in buying patterns.
"Christmas seemed further away for many with fewer people buying Christmas puddings and seasonal biscuits as volume sales were down by 14% and 29% versus this time last year," said Mr Steel.
Meanwhile sales of sun care products more than doubled, and ice cream sales volumes were up by more than a quarter from the same point last year.
Kantar noted that increasing competition between supermarkets was helping to bring food inflation down, with retailers "starting to get the deal stickers out again".
The BRC-KPMG survey also said that shoppers were likely to see stores fighting for custom in the run-up to the key Christmas trading period, and that retailers were "investing heavily to support customers and bring prices down".
"The fight for Christmas shoppers will be fierce this year, with promotions likely to be earlier and abundant in a bid to loosen tight household purse strings," said KPMG's Mr Martin.
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