Asda, the British arm of U.S. retail giant Wal-Mart, reported a pick up in third-quarter sales growth on Tuesday, helped by revamped own-brand food ranges and its pledge to be cheaper than rivals.

Britain's second-biggest supermarket group behind Tesco said sales at stores open over a year rose 1.3 percent, excluding fuel and VAT sales tax, in the three months ended September.

That followed 0.5 percent growth in the second quarter and 0.1 percent in the first.

Asda lagged Britain's grocery market for most of 2010, but has fought back this year, helped by its purchase of smaller format Netto stores and a relaunch of its own-brand food range.

The chain, which completes the conversion of former Netto stores next week, also believes it is benefiting from its price guarantee, which offers to refund customers the difference if an online price comparison web site does not show their shopping was at least 10 percent cheaper than at a rival.

Data from market researcher Kantar Worldpanel last week showed Asda growing faster than the overall grocery market for the first time since early 2010.

Tesco announced a 500 million pound investment in lower prices in September in a bid to lure cash strapped shoppers and stem market share losses.

Asda's underlying sales growth compares with the 2.4 percent increase reported by Wm Morrison Supermarkets over the 13 weeks to Oct 30, a 1 percent rise from J Sainsbury in the 16 weeks to October 1 and a 0.7 percent drop for Tesco in the 13 weeks ended August 27.

Also on Tuesday, Asda's parent company Wal-Mart reported an 8.2 percent rise in third quarter net sales.

(Reporting by James Davey and Mark Potter, editing by Rhys Jones)