LONDON (Reuters) - British grocery sales were flat in the latest 12 week period and all of the country's big four supermarket groups lost market share to the discounters, industry data showed on Tuesday.

Market researcher Kantar said the subdued performance reflected tough comparisons with 2018's strong summer, which was boosted by a heatwave and the men's soccer World Cup. Total sales had fallen 0.5% in Kantar's July data set - the first decline since June 2016.

“The memory of last year still looms large for retailers and this summer’s comparatively poor weather, combined with low levels of like-for-like price rises, have made growth hard to find for retailers," said Fraser McKevitt, head of retail and consumer insight.

Kantar said that of the big four grocers Sainsbury's (OTC: SBRY) was the best performer with a sales decline of 0.6% over the 12 weeks.

"Sainsbury’s will be cheered to be the strongest performer among this cohort for the first time since November 2017," said McKevitt.

Bucking the market-wide own-label trend, sales of branded goods at Sainsbury’s rose by 1.5%, driven by higher levels of promotion and its "price lockdown" strategy.

Sainsbury's proposed 7.3 billion pound ($8.8 billion) takeover of rival Asda (NYSE: WMT) was blocked by the competition regulator in April. Shares in Sainsbury's were up 1.6% at 0730 GMT, paring its losses over the last year to 43%.

Market leader Tesco's sales fell 1.6%, while Asda's (NYSE: WMT) and Morrisons (NYSE: MRW) sales were down 1.5% and 2.7% respectively. Aldi and Lidl's sales were up 6.2% and 7.7% respectively.

(Reporting by James Davey; editing by Kate Holton)

Reuters