The proposals for the Fundraising Preference Service are impractical and could result in public confidence in charities being eroded, the charity chief executives body Acevo has warned.
In its response to a consultation on the FPS, which closed last week, Acevo said the proposals relating to aspects of the scheme such as the data that would be requested from people who wanted to register were unclear and, in order to comply with it, charities might have to use significant resources.
The response said that the FPS had been billed as giving control of who contacts them back to donors, but the scheme as proposed could not stop all forms of fundraising, giving rise to the risk that trust in charities would be damaged.
Acevo said the approach to different fundraising methods was inconsistent because a definition of fundraising communication had not yet been proposed.
"We do not think that this ambiguity is acceptable when there is the possibility that the definition could include activities as wide-ranging as trading communications and lotteries, as well as more traditional donations or asks," the response said.
It said there were also a number of situations in which it was unclear whether charities would be required to screen their contacts against the FPS, such as inviting people to attend events.
It also warned that the "big red button" that would allow people to opt out of all charity fundraising communication could have a disproportionate effect on medium-sized charities because they would be less able than larger organisations to replace the income from lost donors and had less capacity to check the FPS suppression list regularly.
Kristiana Wrixon, head of research at Acevo, said the FPS was "the wrong solution to the right question".
She said: "Members of the public should be able to opt out of communications from individual charities, and charities themselves should move towards a universal opt-in system for communicating with donors.
"However, the model proposed in this consultation is impractical, we do not think that it will be able to achieve its stated outcome and there is a risk that public confidence could be eroded rather than enhanced as a result."
The software company Blackbaud also said the FPS would be unworkable for the majority of charities.