A survey carried out for the consumer group found some people view taking out money in this way as inconvenient, or even a security risk.
The UK Government has said it will legislate to protect the future of cash. Various industry efforts to maintain access are gathering pace, such as being able to request cashback without making a purchase in shops.
Changes made last year have allowed people to request cashback from stores without needing to make a purchase or pay a fee.
But Which? said a survey of more than 2,000 people found just one in six (16 per cent) are aware of the cashback without purchase scheme, and among those who are aware, just under a third (31 per cent) have used it to access cash.
Nearly half (46 per cent) of those questioned said they are unlikely to use the service, with a quarter (25 per cent) of these thinking it will not be convenient.
Nearly a fifth (17 per cent) who are unlikely to use the scheme have concerns about privacy and 16 per cent are worried about security issues.
A quarter (25 per cent) of people who said they are unlikely to use the service said it would feel unfair to the shop or business to handle the cashback service.
Which? said it believed cashback without purchase was a valuable tool in helping to protect access to cash, but it could not meet everyone’s needs and should be viewed as part of a range of solutions.
The consumer group has welcomed moves by major banks to share services to help people and businesses maintain access to cash.
Jenny Ross, Which? Money editor, said: “Schemes like cashback without purchase have a role to play to protect access to cash for those who rely on it, but they won’t be enough on their own to plug the gaps in the UK’s fragile cash system.
“Our research highlights clear limitations of these schemes, with very low awareness and uptake among consumers, and many people viewing cashback as an inconvenient and insecure way to access cash.”
A spokesperson for the trade association UK Finance said: “The banking and finance industry has publicly committed to ensuring there is continued access to cash for those who need it, when they need it.”