German-owned Aldi and Lidl now have a combined market share of 9.7 percent, versus 8.4 percent a year ago, according to the market research company.

Like-for-like sales at market leader Tesco fell 0.9 percent in the 12 weeks to Aug. 16, Kantar said, as strong growth in its convenience shops and online could not offset lower revenue at larger stores.

Asda, which this month said it had suffered its worst quarterly sales performance since it was bought by U.S. group Wal-Mart 16 years ago, saw a 2.5 percent decline and Morrisons was down 1.1 percent.

Tesco's market share fell from 28.8 percent to 28.3 percent year on year, with Asda's share falling from 17.2 percent to 16.6 percent and Morrison's down slightly.

Sainsbury's, which vies with Asda for the second position, eked out a 0.1 percent sales rise, Kantar said, but it was not enough to stop its share of the market falling 0.1 percentage points to 16.3 percent.

Discounters Aldi [ALDEI.UL] and Lidl [LIDUK.UL], whose cheap own-brand goods have forced Britain's big four to slash prices in an attempt to hold on to shoppers, both saw sales growth accelerate to 18 percent and 12.8 percent respectively.

Overall grocery sales grew 0.9 percent, Kantar said, continuing a year-long trend of 1 percent or less growth as record commodity-led deflation also pushes prices down.

Market share and sales growth (percent)

12 wks to 12 wks to pct change

Aug 16, 2015 Aug 17, 2014 in sales

Tesco 28.3 28.8 -0.9

Asda 16.6 17.2 -2.5

Sainsbury 16.3 16.4 0.1

Morrisons 10.8 11.0 -1.1

Co-op 6.4 6.4 1.1

Waitrose 5.1 4.9 3.7

Aldi 5.6 4.8 18.0

Lidl 4.1 3.6 12.8

Iceland 2.0 2.0 3.4

(Reporting by Paul Sandle and Neil Maidment; Editing by Keith Weir)