Simon Edmands has challenged the charity about a letter that tells supporters their direct debits will be increased unless they write back to object.
He has written to the society to complain that this is a "covert" form of fundraising that could alienate supporters and bring in less rather than more income.
Chris Vick, director of marketing and communications at the WDCS, said the method was ethical and might be used again. The initial batch of 2,800 letters generated three complaints.
Megan Pacey, director of policy at the Institute of Fundraising, said she was concerned about the lack of transparency. "They need to be more up-front about it," she said.
Edmands told the charity he had been a fundraiser for ten years. His letter read: "I know that such a mailing, which generates income through inactivity rather than proactivity, is legal, but would question whether it is ethical, especially as there is absolutely no mention at all (deliberately, I wonder?) of the automatic direct debit increase until the second page."
He told Third Sector: "I suspect half the supporters threw it in the bin without realising it would cost them. We've got used to the corporate sector pulling a fast one, but for charities to go down the same route is a bit alarming."