Tesco and Asda's two-hour delivery time slots could be putting them at a disadvantage compared with their rivals, according to research.

Tesco and Asda offer

two-hour slots, while Waitrose Deliver, Ocado and Sainsbury's all offer a

one-hour option.

Nearly three-quarters of 1,000 UK shoppers questioned in an Ipsos MORI poll believed enhancing the flexibility of delivery time slots was the most important change they would like to see to online services.

This was ranked higher than better return and refund policies, which 65% of respondents said was important, and clearer illustrations, which 61% wanted to see. Improvements to online payment security and a customer service helpline to call were important to 56% and 55% of shoppers respectively.

Lengthy slots no longer met the needs of consumers, said Susanne Goller, director of retail at Ipsos MORI, and retailers who still offered them were completely "missing the point" about online shopping. "People buy online to save time - but waiting at home for two hours is an utter waste of time," she said.

The survey also revealed that just one negative online experience was enough to discourage a shopper from ever visiting a retailer's bricks and mortar store again. "Shoppers make no distinction between a retailer's on and offline offers," said Goller. ? "This shows how important it is to make sure the online experience is of a high quality and is consistent with the total brand experience."

Delivery drivers also played a crucial role, she said. "A rude or grumpy delivery driver can have a disastrous effect on a shopper's perception."

Barriers to shopping online included concerns about product freshness, fears products may arrive damaged and the number of carrier bags used.

The survey also quizzed shoppers on the Continent about their online activities, but found UK consumers were ahead of their European counterparts in terms of internet shopping.