Each of the "big four" grocers lost customer share in the last quarter despite increasing sales as Aldi and Lidl collectively accounted for half of the entire market's overall growth, figures show.
Morrisons was the best performer of the four over the 12 weeks to October 8, with sales up 2.8%, followed by market leader Tesco's sales increase of 2.1%, Kantar Worldpanel said.
Sainsbury's sales were up 1.9% and Asda saw growth of 1.8%, but Aldi and Lidl continued to outperform their major competition with sales up 13.4% and 16% respectively to collectively add £390 million.
Christmas appears to have come early for shoppers as they snapped up £69 million of chocolate confectionery boxes, £4 million of mince pies and £1.1 million of Christmas puddings over the past four weeks, Kantar's figures show.
Supermarket sales overall were up 3.1% compared with last year, the 17th period of growth in a row for Britain's grocers.
Like-for-like grocery inflation remained at 3.2% but consumers could be welcoming a slowdown in price rises in the new year, Kantar predicted, with the rate projected to fall below 2% in the first quarter of 2018.
Fraser McKevitt, head of retail and consumer insight at Kantar Worldpanel, said: "Aldi and Lidl collectively added an additional £390 million in sales this quarter, which accounts for half of the entire market's overall growth this period."
Figures from Nielsen for the four weeks ending October 7 show UK supermarkets' sales revenue rose 3.9% year-on-year - the seventh consecutive month of sales growth above 2% - while the volume of goods sold increased by just 1.6%.
Mike Watkins, Nielsen's UK head of retailer and business insight, said: "Supermarket food inflation, at 2.2%, is at its highest for nearly four years and retailers have done a great job of cushioning shoppers from the rising food chain costs they're experiencing, exacerbated by the weakening pound, but this can't last for ever.
"Inflation is helping supermarkets' growth and the good news for them is that shoppers are still spending. Meanwhile, the good news for shoppers is that grocery inflation is still below that in other parts of the economy - such as travel and fuel - and should peak later in the year."