While Aldi is benefiting from the pressure, momentum at Morrisons is declining after last year’s sale, Mr Black said. US private equity firm Clayton Dubilier & Rice won a £7 billion auction for Morrisons last October, but the deal did not receive approval from competition watchdog until June.
“The Morrisons takeover, the amount of time it took the merger watchdog to look into it, it was impossible for the new owners to get into the business,” said Mr Black.
Recent data from rival research firm Nielsen showed that Aldi has already caught up with Morrisons.
Morrisons is in fourth place since the Safeway acquisition in 2004.
Mr Black said: “We will be talking about the reconstruction of the top four for the first time in many decades.”
The German discounters, which started their attack on the British market in the 1990s, now account for 16.1 pc of the total supermarket market. That is an increase of less than 4 percent in 2008.
In addition to switching supermarkets, Kantar found that people are increasingly buying their own branded products to control rising prices. Sales of store-brand products rose 19.7 percent this month.
Tesco and Sainsbury’s have responded to growing pressure on customers’ budgets by promising that Aldi will match the price of hundreds of items.
Other chains have lowered prices and launched cheaper food ranges.
Traditional supermarkets have been accused of being too complacent about the rise of Aldi and Lidl in the past.
Kantar’ McKevitt said: “Over the past month, we’ve seen retailers really expand and promote their own in-store value range to meet demand.
“Asda’s Just Essentials line, launched this summer, is already in 33 percent of its customers’ baskets.”
Kantar found that Aldi and Lidl together achieved 1.8 percentage points of supermarket sales in the past 12 weeks, representing a £2.3bn annual shift in spending.
Asda, meanwhile, returned to growth with sales up 0.2 percent in the 12 weeks to August to bring its market share to 13.9 percent. Tesco’s turnover also increased by 1.0 pc and it has 26.9 pc of the market.
Shares of Sainsbury’s and Morrisons are at 14.8 percent and 9.3 percent respectively, while Waitrose controls 4.6 percent of the market.
Ocado grew by 6.2 pc. because it attracted new customers outside its traditional demographics, although its market share remained stable at 1.8 pc.
Co-op’s sales grew 0.4 percent, giving it 6.5 percent of the market, while Iceland’s grew 2.8 percent, with the frozen food chain accounting for 2.3 percent of the market.