Supermarket shoppers are paying on average 3% more for their weekly groceries even after promotions are included.

The Grocer Price Index, compiled by pricing experts, is the only food inflation tracker that factors in the impact of promotions rather than simply following the base price.

Kantar Worldpanel’s latest grocery inflation figures were 4% [12 w/e 20 March], Nielsen’s 4.5% [4 w/e 19 March] and ONS’s 6.2% [12 months to February]

With a record 40% of grocery purchases on promotion [Nielsen 4w/e 19 March], the GPI figures are a much more representative representation of the market and paint a clear picture of the current price war, particularly between Asda and Tesco.

Asda, which invested heavily in its pricing in January in a bid to bolster its claims to be 10% cheaper than rivals, proved the most successful retailer at keeping inflation in check. Impressively, its pricing is at the same level as a year ago.

And while Tesco’s prices were just 1% higher in March than at the same time last year, Tesco has responded to Asda’s Price Guarantee with its own round of price cutting in February.

Its prices were 2.8% cheaper month-on-month, while Asda’s prices actually crept up 0.1% compared with February.

Sainsbury’s, which has been the star performer of the big four in the past year in terms of sales growth, actually put up its prices the most over the period.

Its prices were, on average, 5% higher than a year ago. And despite its ‘Biggest Ever Price Crunch Campaign’, Morrisons was 4% more expensive than this time last year.

Waitrose, which is not included in the GPI calculations, was 3% more expensive than a year ago but 0.2% up on February.

Despite the continued fears surrounding rising oil prices, food prices actually fell during March - down an average of 0.7%.

This was primarily driven by a 3% fall in the price of bakery lines and a 2.7% fall in chilled goods. The dry grocery category was also an average of 1.7% cheaper in March than during the previous month.