Publication of this month's TradeTrak market share figures coincides with the release of interim results by both Sainsbury and Safeway.
The headline figures posted by both chains were impressive.
Sainsbury's supermarkets business in the UK raised operating profit by 14.9% to £286m and sales by 3% to just over £8bn. Safeway, meanwhile, said its pre-tax profit was up 4% at £188m on sales up 2% at £5.1bn (including the joint venture with BP).
However, as the data below shows, the chains continue see their market shares decline with both suffering the biggest year-on-year falls of the retailers surveyed.
In its statement to the City, Sainsbury boss Sir Peter Davis highlighted the challenging market conditions being experienced in the UK. "There is less growth compared to last year, little food inflation and increased competitive pressures," said Sir Peter.
This deflationary environment is borne out by the data produced for us by ACNielsen which is based on value sales and, therefore, reflects the downward pressure on packaged grocery prices.
Safeway's decision to launch its Go Low, Stay Low price cutting campaign on 5,000 lines will add to that pressure.
Sainsbury also claims its sales performance has been dented by axing Air Miles in advance of the launch of the Nectar scheme. Sir Peter has high hopes for Nectar, which he claims is the UK's largest loyalty scheme. "It's a powerful promotional tool and a mechanism by which we can shape stores and products to meet customer needs."