At least 12 charities are preparing a joint response to raise their concerns about new mail preference service www.itsmypost.com.
The website, set up by the REaD Group, carries a list of the 400 organisations it claims are the most prolific mailers in the UK.
The group of charities, which includes Save the Children, the Church Urban Fund and Amnesty International, have complained that the service has increased their workload.
They have been receiving emails with instructions to remove between 10 and 40 names from their databases each day. In most cases, these names have never existed on their mailing lists, but a record of them would have to be kept to stop any future correspondence being sent to them.
"We are concerned about the huge amount of work involved," said Diane Tye, data protection manager for Save the Children, who has been co-ordinating the response. "Sending these demands less frequently and in a different format would make a real difference."
A spokeswoman for the REaD Group said: "It is a new service and we are trying to ensure it works well for everyone involved."