The proposal is part of the office's draft guidance on section 67 of the Charities Act, expected to come into force early next year. Fundraisers will have to continue to say whether they are paid and, for the first time, state how much. Fundraisers earning less than £5 an hour or £500 a year are exempt.
However, the Public Fundraising Regulatory Association said that stating the precise amount would confuse donors.
"A bleak statement like this would be misleading because it doesn't talk about return on investment, lifetime value or that we spend 5p in a pound on fundraising," said Mick Aldridge, chief executive of the PFRA, who has drafted alternative disclosures to put to the OTS.
The PFRA is working with the Institute of Fundraising, the Association of Charity Consultants, the Charity Commission and the Fundraising Standards Board as part of the OTS's working group on the draft guidance.