The surprise announcement prompted the board of Standard Life to state that the resignation takes effect immediately and that a “search for her replacement has begun”.
During the period of the search, the existing reporting lines to this position will be temporarily reassigned, Standard Life added.
Hunt, who had been in the role for four years, previously worked at Royal & Sun Alliance and Aviva, where she served as chief financial officer at its Norwich Union business. She is also a member of the board at the Association of British insurers.
Earlier this year, Standard Life announced a senior management change with the news that Standard Life Asia and emerging markets chief executive Nathan Parnaby was to leave after more than 30 years working for the company, to be replaced by Alan Armitage.
In its statement, Prudential said the current CEO for its UK and Europe Rob Devey would leave Prudential at the end of October to pursue "new opportunities". Devey was appointed to this role in July 2009.