Age and literacy are factors in people not using the internet, the report says
The one in three Welsh households which lack internet access have been warned they could miss vital information and better deals in goods and services.
A report says they risk failing to find out about such issues as food safety, weather alerts and political decisions.
Consumer Focus Wales wants regulators to ensure vulnerable groups such as the elderly and poor are not forgotten and end up "institutionally locked out".
The Welsh Assembly Government said it was preparing a response.
The report drew on 10 years of data, including work by its predecessor organisation, the Welsh Consumer Council, for its report, Logged In or Locked Out?
The report found 67% of households in Wales have a home internet connection and 94% of these internet connections are broadband.
People aged 35-44 are most likely to have a home internet connection (83%) and 37% use a computer outside the home.
But it concluded that there is a risk of a growing digital divide for the 750,000 adults in Wales who either cannot or choose not to use the internet.
Sarah Richards, senior policy advocate for Consumer Focus Wales, said personal access to the internet was becoming "integral" to lifestyles.
She said: "The growing availability of information and bargains means that those who either cannot, or choose not to use the internet are in danger of being locked out from accessing the best deals, and the most up-to-date information.
"Our research shows that while the debate about "not-spots" is still important, we must not forget the "net-nots" either - those people who either cannot get access, cannot afford access, or simply don't see the benefit of logging on."
The study found a "significant minority" of consumers, 41%, who did not access the internet choose not to do so, while 19% said they did not need it.
But the report concluded that low levels of literacy may also be a deterrent to using the internet.
It cited the Basic Skills Agency suggesting that 25% of the population of Wales have literacy skills expected of an 11-year-old.
It also cited figures suggesting 72,000 people - 4% of the adult population - have the lowest level of literacy, where they can understand short texts and common signs and symbols.
It concluded: "Addressing the educational requirements of people with low literacy levels is essential for achieving greater levels of digital participation."
Ms Richards added: "The benefits to consumers of the internet are many, but we must ensure we do not end up in a country where people are institutionally locked out from information which may affect them - like weather warnings, or finding out information during food scares or looking at what our elected politicians are saying on our behalf."