As British supermarket giant J. Sainsbury prepares to unveil interim results this week, market watchers have highlighted concerns that the group's online shopping operation is struggling, despite a recent revamp of the site.

Customers have reported that Sainsbury's To You, which was relaunched in October, is frustrating to use; overloaded at peak times, has an ineffective search engine and shows a lack of stability in its system.

Sainsbury regulars have also complained about the length of time it takes to complete an online shopping trip. Apparently since the relaunch of the site a 20-minute shopping trip can now take more than an hour.

According to the company, the number of online customers has increased 30% over the last 12 weeks.

Sainsbury is currently outperforming the grocery sector as a whole, posting a better-than-expected 6% rise in like-for-like sales last month, but analysts are cautions in their estimation of the supermarket company's recovery.