The big four supermarkets are under pressure as prices become less of a differentiator for shoppers, according to research from Kantar Worldpanel.

Kantar's grocery share figures for the 12 weeks ending 15 September, 2013, which revealed the purchasing habits of 30,000 households in Great Britain, found that Sainsbury's is the only big four grocer to increase its market share over the past year. It grew from 16.4% to 16.6% and recorded a market-beating growth of 5.1%.

But the other big retailers have all lost market share over the past year, although Asda is ahead of Tesco and Morrisons in terms of its year-on-year sales growth, which stands at 2.4%.

In contrast, Aldi had 32.7% year-on-year sales increases and Lidl and Waitrose also performed well with respective growth rates of 14.3% and 9.7%.

"Strong performances by retailers at both ends of the market pose a significant challenge for the big four supermarkets," says Edward Garner, director at Kantar Worldpanel.

"The combined growth of Lidl, Aldi and Waitrose has taken three market share points out of the grocery market over the past three years and is forcing the major supermarkets to compete for an ever-smaller middle ground."

Garner also argued that price match promotions such as Asda's 'Price Guarantee', Sainsbury's 'Brand Match' and Tesco's 'Price Promise' have meant that cost is less of a differentiator and shoppers cannot be convinced to switch outlets based on cost alone.

There has been a renewed focus on providing high-quality own label products and this strategy seems to be paying dividends.

The upmarket ranges of Tesco Finest and Sainsbury's Taste the Difference have both registered double digit growth as shoppers respond to the premium offer.