Kevin Parry has stepped down after six years as chair of the board at Royal London.
The firm announced it has accepted his resignation, which will see him leave with immediate effect.
He said the group's growth plans means it needs someone who is available for at least another five years.
"It has been a privilege to chair Britain’s leading mutual insurance and asset management group," said Parry.
"The group needs to continue to develop its strategy to take advantage of market opportunities for growth across its insurance and asset management businesses for the benefit of our members.
"That requires leadership from a chair who can commit to a medium-term tenure of five or so years which is not available to me."
Current deputy chair Lynne Peacock, will take on Parry's responsibilities with immediate effect as interim chair while a search for a replacement takes place.
She said: "Personally, I have known Kevin for many years and whilst I know that we will all miss him, we understand the reasons for his decision."
tara.o'connor@ft.com
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