LONDON - Tesco, Britain's biggest retailer, posted a 4.3 percent rise in underlying first-quarter British sales, showing a pick up in growth from the previous quarter but still lagging its main rivals.

The supermarket group, which employs around 470,000 people in over 4,300 stores in 14 countries, said on Tuesday group sales rose 9.7 percent including petrol in the 13 weeks to May 30, including a 20.1 percent rise in international sales at actual exchange rates, or 11.4 percent at constant rates.

Analysts had expected sales at British stores open at least a year to rise 4.3 percent, excluding fuel and VAT sales tax, up from 3.7 percent the previous quarter, according to the average forecast of eight polled by Reuters. Tesco, which takes almost one in every three pounds spent in a British supermarket, has been losing market share for months. It says this is due to shoppers switching to its cheaper discount range, which has boosted sales volumes but meant it has not benefited from food price inflation as much as rivals.

J Sainsbury, Britain's third-biggest grocer, is expected on Wednesday to report a 7.3 percent rise in underlying sales for the 12 weeks to June 13. Industry number four Wm Morrison reported an 8.2 percent rise in underlying sales for the 13 weeks to May 3 earlier this month.

Tesco shares have performed broadly in line with the DJ Stoxx European retail index .SXRP this year. They trade at 12.3 times forecast earnings, below Sainsbury on 14.3 times and international rival Carrefour on 12.6 times.

(Reporting by Mark Potter; Writing by Dan Lalor; Editing by Andrew Callus)