A stream of half-price offers, rollbacks and price crunches, hasn’t prevented prices rising at the big four supermarkets.

The Grocer Price Index showed retail prices were 1.3% higher in February than January and up 15.4% year-on-year. Supermarket prices in the first two months of 2009 increased more than in the same period last year: 3.1% compared with 2.8% in 2008.

Sainsbury's monthly price inflation was 3.0%, versus gains of just 0.8% at Morrisons and Tesco. Asda prices were up 0.9%.

Falling exchange rates, delays in the effect of falling commodity prices and last summer’s wet weather are responsible for much of the increase, but one analyst said downtrading might also be a factor.

“Premium items often have higher margins, so if less are selling, something has to offset that,” he said.

Retailers said customers were taking advantage of promotions and multibuys in greater numbers, reducing their typical shopping bills. Take-up on discount coupons is also reported to be higher year-on-year.

In February, the biggest price rises were in alcoholic drinks, ambient grocery and frozen foods, while dairy, meat and fish, bakery and confectionery fell.