Ocado provided the best service of all the online grocers over Christmas - and Tesco the worst - according to research carried out exclusively for The Grocer.

Mystery shoppers used by eDigital Research placed 36 orders with the major online retailers in the week beginning 17 December and rated their experience based on 10 criteria, including first impressions, search facility, range, shopping basket, purchasing and delivery.

Ocado was rated best overall, achieving the top score in half the categories. It provided the best search and shopping basket facilities, as well as the best customer service contact via telephone and email.

Tesco's search facility ranked worst of all the online grocers with a 76% satisfaction rating, compared with Ocado's 96%, Sainsbury's 86%, Asda's 83% and Waitrose's 79%.

Tesco also attracted poor ratings for its product range. Its 81% satisfaction score was beaten by Asda's 91%, Ocado's 88% and Sainsbury's 84%. Only Waitrose scored lower, with a 74% rating.

Asda also scored highest on first impressions of the website, which helped the supermarket come in second place overall, indicating the work it has put in to improve its online service over the past year is paying dividends. Sainsbury's came in third, ahead of Waitrose in fourth place.

Steve Brockway, eDigital research manager, said he was surprised at Tesco's poor showing.

"The Tesco site generally does quite well in our mystery shops, so I don't know what was going on this Christmas," he said. "Some shoppers said keyword searches brought up irrelevant results."

All the online grocers, including Tesco, scored highly on service and site quality in comparison with non-food retailers, according to eDigital Research.

"Customers were not let down on deliveries, which largely arrived when promised, an area other online retailers did not do so well in," said Brockway.