The detailed in-home survey of almost 2,000 Britons found that 18% described themselves as non-users.
The Oxford Internet Institute (OII), which carried out the research, said people falling into the category tended to be older and poorer than frequent net users.
The size of the group presented a “dilemma” for any government trying to reach and support them, said the OII.
“Non-users are older, proportionately less well-educated and have lower incomes,” said Dr Grant Blank, survey research fellow at the OII, who oversaw the project.
Non-users, said Dr Blank, were those who did not go online via any means – either phone or computer. The proportion of those falling into this category grew as people aged, he said.
The figure of 18% is higher than other official measures of non-users, he said, because of the way the OII sampled the UK population.
Figures gathered by the Office for National Statistics suggest about 7.4% of the population are non-users but this figure is drawn from data gathered for its Labour Force Survey.
This, said Dr Blank, looked at people who were in work, or might be, and excluded those who did not work, as well as other non-workers such as people with long-term disabilities or residents of care homes.
By contrast, he said, the OII survey visited people in their homes and gathered a more detailed view of Britons’ online lives.
“Virtually everyone is online before age 50,” Dr Blank told the BBC.
After that milestone, he said, there was a “sharp decline” in use of about 2% a year.
“There are a lot of things about the internet that get less useful as you get older,” he said.
As people’s circle of friends and interests shrink they see less and less reason to spend time online.