Almost a million adults do not have a bank account, according to the government-backed watchdog, Consumer Focus.
It has issued a major report, On the Margins, looking at why so many people exist solely in the cash economy.
The study comes as the government launches a reform of mainstream banking.
Consumer Focus is calling for "the real lives and experiences of consumers to be at the heart of these changes".
The organisation, made up of a merger between Energywatch, Postwatch and the National Consumer Council, says that those who do not have the facility miss out on a range of benefits that many people take for granted.
They pay more by not being able to use direct debit for bills - or the internet to buy goods and services.
Getting money in the first place is hard, too, with fewer employers paying staff in cash. It is estimated that by 2018, only 2% of employees will be paid in this way.
It based its study on 50 in-depth interviews with consumers without a bank account, including people in difficult circumstances such as bereavement, bankruptcy or homelessness.
The report identifies a number of common barriers that prevent consumers taking advantage of the benefits of mainstream banking services, including language problems and existing debts.
Some, of course, simply do not want an account, for whatever reason.
One way forward for those who do, says Consumer Watch, could be to build on the Post Office Card Account (Poca), which is used by claimants to withdraw their benefits by card.
Although that is all these cards do, they do mean there is already a spring-board from which other simple services could be added, such as an ATM card and a savings option.
Time and money
Mike O'Connor, chief executive of Consumer Focus, said he accepted that there was work to do to make it worth banks' while to help create accounts for such people.
"Banks are not charitable institutions, but they are a necessary gateway to the benefits of an increasingly online and cashless economy. Life without a bank account can cost time, money and convenience," he said.
Some question whether much further help is necessary.
A statement from the British Bankers' Association (BBA) said it was important to note that not everyone wanted a bank account.
It said: "There is a limit to how many basic accounts banks can open as it is as much a question of demand as much as supply.
"Many people are comfortable operating in cash as they feel it helps them budget better. And their choice to operate in cash should not be seen as inferior."
It added that banks were opening about 35,000 basic bank accounts a month and had met the previous government's target for new accounts last October.