Sainsbury's counts the cost of its website crashing

Sainsbury's was counting the cost of another day of its online grocery service being down, as its rivals started an offensive to grab disgruntled customers.

Sainsbury's has not made any deliveries to customers since the site first went down on the evening of 17 June. Yesterday, the UK's third-biggest grocer was not able to confirm when the site would come back up, as The Independent went to press.

Given that Sainsbury's delivers to 90,000 customers a week and their average spend is thought to be about £80 per transaction, the technical glitch has already cost the grocer well over £1m in lost online sales.

Sainsbury's has also offered £10 compensation to an estimated 20,000 people affected by the problem over Wednesday and Thursday, which will add another £200,000 to the retailer's online grocery bill.

However, a Blue Oar analyst, Greg Lawless, said that Sainsbury's may feel a bigger impact from customers deserting its service.

He said: "They have to win back the trust of customers and this is not something that will happen overnight and they have let customers down in a very bad way."

The embarrassment is particularly acute for Sainsbury's, which trumpeted the growth of its online site in its first quarter results presentation on 18 June, when Sainsbury's chief executive, Justin King, said: "The online operation is continuing to perform well, with sales growth at over 40 per cent."

Rivals Asda, Tesco and Ocado have all cheekily moved to grab customers away from Sainsbury's. An Asda spokeswoman said: "We have seen a five per cent uplift in our home shopping orders in the last 24 hours as Sainsbury's customers 'try someone new today'."

Asda started running a Google advertisement on 18 June, when it realised Sainsbury's site was down, offering them free delivery on home shopping when they enter a promotional code.

A Tesco spokeswoman said: "Tesco.com would like to reassure all Sainsbury's online customers that they needn't be without their groceries." Tesco, which typically serves 300,000 online grocery customers a week, is offering new customers £5 off when they spend more than £50.

An Ocado spokesman said: "We have brought in extra drivers and opened up more delivery slots so no one misses out."