Boots, the UK's biggest chemist chain, has the "least available" website out of the 49 most popular UK retail websites, according to tests conducted by server monitoring software provider Uptrends.
Boots.com is primarily an online shopping website for health and beauty products, but also contains advice on health issues for the general public.
It was one of 49 websites monitored, which included the likes of NetFlix, ASDA, John Lewis, House of Fraser and Top Shop.
Uptrends monitored the websites from September 8 to October 11 - a period of 33 days - and it found that 17 of the websites did not meet the industry standard of 99 per cent availability.
The 17 included The Carphone Warehouse, Currys, Travelodge UK, ASOS and Debenhams, with uptime ranging between 99.8 per cent and 98.4 per cent.
TheTrainline had an uptime of 98.18 per cent - resulting in more than 14 hours of downtime in 33 days.
But none of the retailers aside from Boots, with 96.77 per cent, went below the 98 per cent mark. Boots had 1559.2 minutes of downtime in the 33 day period.
An Uptime spokesperson explained that the firm tested all of the homepages of the websites every five minutes to check whether they were available from all of its "checkpoints" around the world.
"If one checkpoint discovered that a website wasn't available, a second checkpoint checked the website again. If this checkpoint also confirmed that the website was unavailable, this was marked as ‘downtime'. This could be caused by problems with the server, technical issues or perhaps excessive maintenance on the website, which causes it to be unavailable," the spokesperson said.
Boots.com had several hours of downtime on 14 September, 20 September, and a 12-hour period on 8 October.
Meanwhile, 14 out of the 49 websites had 100 per cent uptime during the testing period. These included Amazon.co.uk, Apple.com, Tesco.com, Ikea.com and Marksandspencer.com.