The consumer group's members found that of 16 home and garden websites, B&Q's diy.com was least successful. The DIY/garden retailer's chief executive Ian Cheshire appeared on video at the recent HTA Garden Futures conference saying people use websites "to make their lives more efficient".
But Which? readers said: "B&Q's site could be improved, especially the categories of products, which would help searching for things."
The site received just one star out of five for website navigation. Homebase scored 62 per cent and came 14th in the table, also scoring one star for product range.Thompson-morgan.com scored 63 per cent (13th), with Crocus 11th on 68 per cent and gardening4less.co.uk in eighth position with 73 per cent. John Lewis, Screwfix, Amazon and Lakeland were in the top five.
Which? researcher Sarah Dennis said: "Readers said the B&Q search function was hit and miss. Thompson & Morgan scored highly for range but, like Crocus, not for price. Search functionality and value for money are what consumers are looking for."
A B&Q representative said: "B&Q is disappointed that diy.com has not performed better when Which? members rated online shops.
"We continue to make significant investment in our website and listen to our customers when making design and development improvements.
"We've recently refreshed our site to include a wealth of new features like a next-day product delivery service and a 'reserve online and collect in store' service.
"One of the things people love about our site is its inspirational content and interactivity, enabling those embarking on home-improvement projects to get everything they need to successfully complete the job - from tips and expert advice to the actual products to do the job."
The representative added: "We ask more than 1,000 visitors to our site every week for their views and these are overwhelmingly positive.
"As a result of our recent improvements, we've been shortlisted for numerous awards this year and have significantly improved our positioning in the annual Webcredibles research, which ranks online shops of high-street retailers."
1. Johnlewis.com - 91%
2. Screwfix.com - 88%
3. Amazon.co.uk - 88%
4. Lakeland.co.uk - 87%
5. Amazon.com - 84%
8. Gardening4less.co.uk - 73%
11. Crocus.co.uk - 68%
13. Thompson-morgan.com - 63%
14. Homebase.co.uk - 62%
16. Diy.com (B&Q) - 59%