Morrisons has beaten Tesco on price to become Britain's second-cheapest supermarket behind Asda over the past six months.

While Asda's average weekly basket came out cheapest at £44.51, the half-year review of The Grocer 33 shows Morrisons' basket cost £45.65 compared with Tesco's £46.41. It is the first time Morrisons has trumped Tesco on price over a prolonged period since our survey began in 1997.

It is also the third price coup Morrisons has pulled off at Tesco's expense in one week. The retailer had a cheaper trolley of 100 items in The Grocer 100 Price Index this week, which has happened only once before in the past six months. And last week it had a cheaper basket in The Grocer 33's first-ever Christmas pricing survey.

"We've worked hard to offer great prices and exceptional promotions," said Martyn Jones, Morrisons' trading director.

Morrisons also retained its title of top of the stocks with an availability level of 97.86%. Tesco, however, insisted it was still the cheaper of the two. "We urge customers to look at our online price checker where they will see Tesco is cheaper than Morrisons on more than 2,000 products," said a spokeswoman.

Asda has spent 20 of the past 27 weeks as cheapest retailer in our weekly surveys - and is on an 11-week unbeaten run. "We've always said low prices are part of our DNA," said Asda chief executive Andy Bond. "Having the lowest basket price over the past six months proves it."

Waitrose was best for customer service over the six-month period after winning the top store of the week award six times - twice the number of times it earned the plaudit during the same period last year.

This reflects efforts in cutting queue times to four minutes and 46 seconds - the fastest of all the retailers - and improving availability to 97.38%, consigning Tesco to third place behind Morrisons and Asda.

"We continue to push new boundaries through our dedication to service," said director of selling Tony Solomons.