Doing your grocery shopping online can be slow and unreliable, according to a study by consumer campaigners Which?.

Which?Online sent 25 volunteers to the Asda, Iceland, Sainbury's and Tesco websites. Most were disappointed, finding that online shopping saved them little, if any, time compared with their normal shopping trips.

Over half the orders took more than an hour and a half to complete, mainly because the shoppers had problems finding goods and sites crashed fairly regularly.

Many volunteers couldn’t locate everyday products and, even when they did, were not told if the item was in stock when ordering. The mystery shoppers were unsatisfied with replacement goods sent out and complained that many mistakes were made with their orders.

“Supermarkets should make more of an effort to keep everything listed on their site in stock,” said Simone Collins, who compiled the report for Which?Online. “[People should not be] left short of items they’ve ordered or with unacceptable substitutes.”

“Replacement goods are as near to the requested item as possible,” said a spokesperson for Asda. “It would be impossible to constantly keep track of all produce on the shop floor and notify users immediately that stock was out of order.”

Asda said any unsuitable goods could be returned - at the customer's expense.

Worse, most of the shoppers were sent items with imminent ‘use by’ or ‘best before’ dates.

“Fresh food should be just that. Fresh,” said Collins. “We were actually sent a stale loaf.”