The government is refusing to tell council tax protestor Sylvia Hardy who paid off her bill which led to her early release from prison.
Sylvia Hardy has said she is willing to go to jail again
Ms Hardy, from Exeter, Devon, went to prison in September for failing to pay £53 of last year's tax.
The 73-year-old spent two nights in jail before the benefactor stepped in.
The National Offender Management Service has told her they cannot disclose the person's details under the Freedom of Information Act.
Former social worker Ms Hardy can appeal against the decision.
She faces another jail sentence after she appeared in court on 24 October and was told to pay this year's bill.
Exeter magistrates ordered Ms Hardy to settle the £323 she owes on her 2005-06 tax bill, plus £30 costs, within 28 days.
She told the court that over the past four years her occupational pension had risen a total of 6.8% as against a total increase of 38.1% in council tax for her two-bedroom flat in the city.
The pensioner said if she was imprisoned again she wanted to serve her full jail term.
She has appealed to well-wishers not to pay her outstanding tax.