Sainsbury’s faces an increasing fight for shoppers’ spending in its London and Home Counties heartlands from discounters such as Aldi and Lidl.

According to data seen by The Times, nearly 1,000 locations that are capable of supporting a discount supermarket, many of which are in the South East, have been identified by CACI, the market research company.

The sites are disproportionately found in the areas of the South where the hard discounters are yet to penetrate. About £11 billion of Sainsbury’s £17.8 billion revenue comes from areas where it could soon be in competition with Aldi and Lidl, CACI calculates.

Increasingly discounters are targeting the middle-class customers on whose loyalty Sainsbury’s has relied. The market will be examining Sainsbury’s second-quarter trading update this week for evidence of vulnerability as consumers prepare for a recession.

Tesco is even more exposed to the advance of the discounters: an estimated 78 per cent, or £20 billion, of its £25.6 billion turnover is earned from areas where Aldi and Lidl provide no direct competition.

Morrisons and Asda are less vulnerable because their northern bias means that they are already exposed to competition from Aldi and Lidl, yet they still face a fierce threat.

Asda, the second-largest supermarket by market share, receives £11 billion of sales from areas free of discount competition, but 40 per cent of Asda’s sales derive from areas where the discounters have already set up.

Paul Langston, an associate director at CACI, said that the “big four” of Tesco, Sainsbury’s, Asda and Morrison faced a guerrilla war as discounters benefited from a willingness to set up on sites that their larger rivals would consider inappropriate. The bad news for Tesco and Sainsbury’s is that Lidl and Aldi are losing the stigma that once made them out of bounds for middle-class shoppers.

Mr Langston said: “We feel that the customer profile of the discounters has levelled out and is much more representative of the UK as a whole.”

Tesco has most to lose: it commands 30 per cent of the market. Aldi, Lidl and their rival discounter Netto account for only 6 per cent of the market between them, but Aldi says that sales rose by 25 per cent last year. Lidl posted similar year-on-year growth.

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