The data from market researcher Nielsen said Tesco's market share was 27.8 percent in the period, down from 29.2 percent in the same period last year.
The Nielsen data showed a greater slowdown for Tesco than the 3.6 percent decline reported by Kantar Worldpanel on Tuesday.
On Thursday Tesco reported a slump in first half profit and said accounting transgressions went back further than initially thought.