Asda and Morrisons outgrew their big four rivals last month as shoppers started stocking up on Christmas essentials.
The Yorkshire-based grocers both enjoyed 2.4% sales uplifts during the 12 weeks to October 7.
The Co-op was the only other bricks-and-mortar grocer to expand its market share during the period, following a 7% jump in sales.
Overall grocery sales posted a 3.2% uplift year on year to £26.2bn.
Kantar Worldpanel’s head of retail and consumer insight Fraser McKevitt said the onset of colder weather and darker evenings “inspired customers” to turn to Sunday roasts and stock up on Christmas staples.
Shoppers spent £4m on mince pies alone last month, Kantar said.
Rival data from Nielsen anointed Asda as the best performing member of the big four, with 2.9% sales growth during the 12 weeks to October 6.
According to its research, Morrisons was just behind with a 2.2% uplift, while Tesco and Sainsbury’s grew sales 1.2% and 0.6% respectively.
Growth in overall grocery spending slowed to 1.9% during the shorter four-week period, which Nielsen attributed to shoppers “tightening their purse strings” ahead of the festive season.
Nielsen’s head of UK retailer insight Mike Watkins described it as an “objectively disappointing, though not entirely unexpected, slowdown in sales”.