She said her reasons were a combination of her workload, her health - she has Parkinson's disease - and her frequent transatlantic trips to see her fiance, a wealthy Canadian mining engineer.
"I didn't feel I could do everything justice," she said. "What I wanted to do was keep going until the Charities Bill was on the statute book, but now it seems likely that it won't be until Christmas. And I can't go on crossing the Atlantic because it unbalances my medication."
Peacock said her main intention had been to transform the commission into "an enabling regulator rather than a hit-them-on-the-head regulator.
"There is now a good clear vision of what we want to do, and I like to think we have achieved a lot already."
She emphasised that she was not planning to retire and would continue to serve on the boards of the Harvard Business School, the Said Business School in Oxford and the Commission on Unclaimed Assets.
She also plans to write about global governance and funding issues and to do mentoring work in the voluntary and private sectors. Adverts for her replacement will be published next Sunday, with interviews due to take place in February.
- See Newsmaker, page 11.