UPDATE 1-Britain's Asda reports first sales growth in three years

(Adds detail, Wal-Mart, Asda CEO comments)

LONDON, Aug 17 (Reuters) - Asda, the British supermarket arm of Wal-Mart, the world's largest retailer, on Thursday reported its first underlying sales growth for three years, helped by food price inflation and very weak comparative numbers last year.

Asda, the UK's third largest grocer after market leader Tesco and No. 2 Sainsbury's, said like-for-like sales rose 1.8 percent, excluding petrol, in its fiscal second quarter to June 30.

That compares with a 2.8 percent decline in its first quarter, which was an eleventh straight decline.

In Asda's second quarter last year sales slumped 7.5 percent - its worst ever quarterly result.

"Customers are responding to investments in price and store experience by visiting the stores more often and increasing their basket sizes," said Wal-Mart President and CEO Doug McMillon.

"Theres still much more to be done, but were clearly headed in the right direction," he said.

Asda CEO Sean Clarke noted 275,000 new customers chose to shop at Asda in the second quarter, particularly during Easter, but he said the market remained competitive.

Of Britain's big four supermarket players, which also includes Morrisons, Asda has been hurt the most by the rise of German discounters Aldi and Lidl as its traditional price advantage was eroded.

Wal-Mart has said Asda was too slow in responding to that competition.

All of Britain's grocers are also having to deal with cost pressures as the post-Brexit vote fall in the value of the pound has made imports more expensive, and with more grocery sales moving online. (Reporting by James Davey; editing by Kate Holton)