Asda has overtaken rival Sainsbury's to become the UK's second biggest supermarket chain.
Asda sales have leapfrogged Sainbury
Asda has recently gained 17% of the market - up from 16.1%, while Sainsbury's slipped from 17.1% to 16.2%, industry group TNS Superpanel said.
The news has been widely expected since 1999, when the chain was taken over by US group Wal-Mart.
Sainsbury's had a firm grip on the top spot until 1995 when it was overtaken by Tesco.
Asda said: "We're obviously pleased that this month's TNS figures show we've nipped ahead of Sainsbury's for the first time.
"But while we're pleased more and more customers seem to like our prices and service, overtaking Sainsbury's has never been our goal.
"We'll only continue to grow and be successful if we keep focusing on the individual needs of our customers, store-by-store."
Robert Clark of the analysts Retail Knowledge Bank said the revelation had been expected for some time.
Asda v Sainsbury's
Market share 17% v 16.2%
Grocery share 13% v 15.5%
Number of shops 260 v 500
"Clearly Asda has been one of the winners along with Tesco and Morrisons in the supermarket race and Sainsbury's has been one of the losers.
"Asda has an inbuilt advantage, in that its stores are larger than average size and its been filling them with a lot of non-foods.
"That has enabled it to overtake Sainbury's," he added.
Sainsbury's chief executive Peter Davis will not have been too shocked by the news.
Last week, he admitted he was disappointed with sales growth, and in October he warned Asda was breathing down its neck.
But, Sainsbury's did retain its lead over Asda in the food-only sales stakes. About 20% of Asda's sales are non-food, compared with 10% for Sainsbury.
It added that in terms of food sales the chain has a 15.5% share of the grocery market, while Asda has 13%.
Sainsbury's said: "The key point is that we are still the second largest grocery retailer in terms of food.
"We know that, we're addressing it and we're relaunching all our non-food in the autumn."
TNS said Asda overtook its rival for the first time in the four weeks up to 20 July.
Supermarket sales by market share (TNS data)
Tesco - 27%
Asda - 17%
Sainsbury's - 16.2%
Safeway - 9.2%
Morrisons - 6%
The analyst normally warns monthly data is volatile, but said the latest figures were part of a clear long term trend and it expected a similar result from its more commonly used 12-week figures.
TNS's Edward Garner said: "This is a considerable shake-up at the top of the British supermarket rankings.
"Asda's growth is reflective of the recent success of supermarkets promoting themselves on price promises."
The chain's George clothing range has proved successful at luring shoppers away from traditional high street stores - so much so that owner Wal-Mart has now exported the brand to the US.
Elsewhere in the supermarket league, Tesco retains the top spot with 27% of the overall market share.
Holding onto fourth place in the table is Safeway - currently the subject of a five-way takeover battle - with a share of 9.2%, down from an earlier 9.9%.
Meanwhile, Morrisons - one of the bidders for Safeway - boosted its share from 5.7% to 6% to hold onto fifth place.