More than one in ten people (seven million) have never used the internet, including over three million over-75s, according to figures published by the Office for National Statistics (ONS).
The ONS said those who are less likely to have used the internet include both the elderly and disabled adults.
By the third quarter of 2013, there were 3.8 million disabled adults (as defined by the Disability Discrimination Act) who had never used the internet - representing over half (54%) of the 7 million who had never been online.
Similarly, 46% of the total number of non-internet users were aged 75 and over, equating to 3.2 million people. Moreover, the number of over-75s who have never used the world wide web is rising, the ONS said. Since the third quarter of 2011, the proportion of older people who have ever used the internet increased by five percentage points.
In total, 43.8 million adults (86%) in the UK had used the internet, an increase of one million since the same period last year. Among 16 to 24 year olds, that figure rises to 99% that have been online.
Jeh Kazimi, founder of over-55s tablet computer service Breezie.com, said: "Having access to the internet and being able to access the internet are two different things. These stats show just how big a barrier age is to internet access.
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"The fact that a third of over 75s have still never used the internet is a major cause for concern. More rural areas, such as East and North Wales and Northern Ireland, are also clearly less connected than other areas of the UK.
"The very people who could benefit most from internet access are the least connected. For the elderly, the internet could vastly improve their quality of life if they knew how to order online shopping, access health advice, save money on utility bills and combat loneliness by connecting with others."