According to the British Retail Consortium (BRC), the U.K. shop prices declined by 1.8% year-on-year in May, after a 1.7% decline in April.

The decline was mainly driven by a drop in non-food prices, which plunged 2.7% year-on-year in May.

Food prices fell at an annual rate of 0.3% in May.

"Looking slightly longer term we know that the recent commodity price increases will start to put pressure on retailers to raise their own prices. We would normally expect these input costs to filter through to prices eventually, but the big question is how far fierce competition in the industry will insulate consumers from price increases," BRC Chief Executive, Helen Dickinson, said.