The business remains by far Britain's biggest supermarket with a 30.2% market share in the 12 weeks to September 15 but the figure was 30.9% a year earlier, according to data from Kantar Worldpanel.
It is one of the "big four" supermarkets - which also include Asda, Sainsbury's and Morrisons - faced with the dual threat posed by Waitrose at the top end of the market and discount stores Aldi and Lidl at the other.
Tesco has recently tried to bolster its upmarket credentials by a promotion for its premium Finest range, sponsoring ITV period drama Downton Abbey.
The brand saw double digit growth over the period - as did Sainsbury's Taste the Difference offering - but it failed to halt the march of Waitrose, which improved its market share from 4.6% to 4.9%.
Tesco, due to announce interim results at the start of next month, is under pressure to improve amid a £1 billion turnaround plan. Its latest trading update showed like-for-like sales fell 1% in the first quarter to May 25.
Meanwhile Aldi rose from 2.9% to 3.7% and Lidl was up from 2.8% to 3%.
Edward Garner, director at Kantar Worldpanel, said: "Strong performances by retailers at both ends of the market pose a significant challenge for the big four supermarkets.
"The combined growth of Lidl, Aldi and Waitrose has taken three market share points out of the grocery market over the past three years and is forcing the major supermarkets to compete for an ever-smaller middle ground."