Budget supermarket Iceland has overtaken Waitrose and Ocado to deliver the best online service, according to a customer satisfaction survey.
While the frozen-foods specialist - which offers free delivery for orders over £35 - topped the online list, Co-op shops ranked worst for in-store satisfaction - for the third year in a row.
Waitrose was number one for its physical stores for the second year running in the annual Which? supermarket survey, scoring a customer satisfaction rating of 75%.
The upmarket chain fared well for the quality of its own label products, staff helpfulness and store appearance.
But Co-op, scoring 57%, frustrated consumers for failing to have products in stock, being poor value for money and having a small range of products.
A spokeswoman for Co-op described the survey as "backward-looking" and insisted service has improved.
She said: "This backward looking survey does not reflect the massive strides we have taken recently.
"Our prices are lower, our produce is fresher and our products more exciting than they have ever been, and that's why the latest market share data out last week showed Co-op as the fastest-growing and most frequently visited supermarket."
Tesco, the UK's largest retailer, came second from last, disappointing shoppers with stock availability and staff helpfulness.
The poll of 7,009 shoppers, carried out in November, showed Asda as having the worst online service, with customers scoring it 65%.
This was contrasted with Iceland's service, which was awarded a rating of 77% for offering the best value for money as well as a high-quality own-brand products.
Iceland's founder and chief executive Malcolm Walker said: "Iceland pioneered home delivery in the UK 20 years ago, and launched the first nationwide online shopping service in 1999, so I can say with confidence that we have more experience in this area than anyone else.
"Since we relaunched online in 2013 our service has grown rapidly thanks to our easy-to-use website, great products and brilliant people - and we now make around nine million home deliveries every year."
He said the store was proud to offer everything from pizzas and prepared meals to quinoa to ostrich steaks thanks to the "power of frozen".
Which? director of campaigns Alex Neill said: "While value for money remains a high priority, people want special offers to genuinely be special and they want a pleasant in-store shopping experience.
"When it comes to online shopping, we know convenient and low-cost delivery slots are prominent factors in where people choose to shop."