Senior independent director Rudy Markham will lead the nominations committee process for the identification and appointment of a new chairman.
"I expect our Annual General Meeting in May 2016
to be my last as chairman," Stewart said in a statement issued by the company.
"We are now coming to a stage where the post-financial crisis work has been successfully delivered: the company is now focused on the right markets in the UK and internationally and well prepared for the transition to Solvency II.
Insurers are waiting for new European capital rules to come into force in January. The Solvency II rules are considered more stringent than existing regulation.
The Bank of England is expected to review insurers' internal models for measuring capital next month.
During Stewart's tenure the group has had a total shareholder return of 399 percent, the company said.
Stewart is a member of the Court of the Bank of England and was previously a non-executive director of the Financial Reporting Council.
(Reporting by Emiliano Mellino; editing by Jason Neely)
