Online shopping is one of the better-performing parts of Britain's retail sector and has become a key battleground for the big supermarkets as they grapple with the growth of German discounters Aldi [ALDIEI.UL] and Lidl [LIDUK.UL].

British shoppers are increasingly choosing the convenience of ordering online and collecting from stores rather than waiting at home for deliveries.

Sainsbury's, however, is playing catch-up with rivals Tesco and Asda, the British arm of Wal-Mart Stores.

Its Click & Collect groceries service, launched in March last year, offers 100 collection sites but is now targeting 200 sites by May 2017, the company said on Tuesday.

The chain's "drive-thru" service allows customers to pre-select a collection time and collect shopping from a refrigerated van parked in a designated area of a store’s car park without having to leave their cars.

Market leader Tesco provides click-and-collect groceries from 330 stores, while Asda provides the service from 618 sites.

The news on Click & Collect from Sainsbury's comes a day before it publishes full-year results.

Analysts are on average forecasting an underlying pretax profit of 574 million pounds ($843 million) for the year to March 12, down from 681 million pounds in 2014/15.

Sainsbury's has shown greater resilience to competition from the discounters than its rivals and last month agreed a 1.4 billion pound takeover of Argos-owner Home Retail.

($1 = 0.6810 pounds)

(Reporting by James Davey; Editing by David Goodman)