The department store business said total sales slipped 1.4% to £78.1m in the week ending June 17.
Fashion sales dropped 3% compared to the same week in 2016, impacted by differing timings of a competitor’s promotion.
John Lewis matched those prices as part of its “never knowingly undersold” mantra.
Despite the overall decline in fashion, the retailer said the beauty sub-category posted “strong” sales, rising 7.9% during the week. Warm-weather clothing performed particularly well, registering a 17% uplift.
Home sales were down 5.2%, although the warm weather drove a 55% spike in sales of outdoor living products year-on-year.
Electrical and home technology revenues rose 5.4%, buoyed by new releases from tech giant Apple and a 1,000% surge in sales of fans.
By contrast, sister retailer Waitrose enjoyed an increase in sales during the same seven-day period, driven by the heatwave.
The upmarket grocer reported a 6.1% jump in revenues, excluding fuel, to £136.3m.
Sales of rosé wine more than doubled, as the favourable weather and a ‘25% off three bottles of wine’ promotion combined to woo shoppers.
Other alcohol lines also sold well, with cider, spirits and beer up 36%, 27% and 22% respectively.
Barbecue food also proved popular as sales in the category were on fire at 114%, while fresh produce also registered a 7.5% climb in revenues.
Waitrose singled out melons and strawberries among the top performers in fresh, growing sales by 33% and 18%.