UPDATE 2-Britain's Sainsbury's underperforms rivals again in latest data - Kantar

* Sainsbury's sales down 0.6% in 12 weeks to June 16 * Overall UK grocery sales up 1.4% in period * Sainsbury's due to update on first quarter next week (Adds table, analyst comment, shares) LONDON, June 25 (Reuters) - British supermarket chain Sainsbury's underperformed its big four rivals again in the last 12 weeks, in a market that posted only modest growth during a wet start to the summer, industry data showed on Tuesday. In the 12 weeks to June 16, Sainsbury's sales fell 0.6%, market researcher Kantar said. In contrast, sales were flat at market leader Tesco , slipped 0.1% at Asda and dipped 0.5% at No. 4 Morrisons. However, all the big four lost share to German-owned discounters Aldi and Lidl, whose sales grew 9.3% and 7.5% respectively. Sainsbury's, which had its proposed 7.3 billion pound ($9.3 billion) takeover of Walmart owned Asda blocked by the Britain's competition regulator in April, is due to issue an update on first quarter trading on July 3. Sainsbury says Kantar underestimates its general merchandise market share, saying it has streamlined its general merchandise range in favour of the range offered by Argos, which it acquired in 2016, and Kantar does not capture Argos' sales. Kantar said Sainsbury's halved the rate of sales decline it registered last month as it attracted more affluent shoppers. Sainsbury's shares, which fell 4% on Monday, were down another 0.4% on Tuesday, extending losses for the year to 40%. Shares in Tesco and Morrisons both fell more than 2%. Bernstein analyst Bruno Monteyne said Sainsbury's performance was its best since January. "Whilst Sainsbury's till roll is still falling, this is clearly a step in the right direction. A few more of these data points could spark a new wave of optimism," he said. Sainsbury's Chief Executive Mike Coupe is under pressure to show the group can prosper on its own after the Asda debacle. He said in May he would improve stores, cut prices on daily essentials and invest in online to restore sales growth. TOUGH COMPARATIVE Kantar said overall UK grocery sales were up 1.4%. "The modest level of current growth is thanks in no small part to the wet start to the summer, with last year’s heatwave and the run up to the men's FIFA World Cup making 2018 a difficult year to top," said Fraser McKevitt, head of retail and consumer insight at Kantar. Grocery inflation over the 12-week period was 1.0%. Kantar said prices were rising fastest in markets such as crisps, bottled cola and dog food, while falling in instant coffee, fresh bacon and detergents. Market share (%) 12 wks to 12 wks to pct change June 16, June 17, in sales 2019 2018 Tesco 27.3 27.7 0 Sainsbury's 15.3 15.6 -0.6 Asda 14.9 15.1 -0.1 Morrisons 10.4 10.6 -0.5 Aldi 7.9 7.4 9.3 Co-operative 6.2 6.1 3.0 Lidl 5.7 5.4 7.5 Waitrose 5.0 5.1 0 Iceland 2.1 2.2 0.6 Ocado 1.3 1.2 11.3 Source: Kantar ($1 = 0.7828 pounds) (Reporting by James Davey; Editing by Kate Holton and Edmund Blair)