One of the most senior women in Whitehall is leaving to head a charity.
Dame Helen Ghosh, 56, will stand down next month as permanent secretary at the Home Office to take up the role of director general of the National Trust.
After a 33-year career in the Civil Service, Dame Helen said she felt "very torn" about going and would do everything she could to leave the department in the best possible shape for the future.
"I have been privileged to lead the Home Office and feel very torn about leaving," she said.
"But the work of the National Trust is something that is very close to my heart and this was a rare opportunity to lead a major voluntary organisation which I could not allow to pass by.
"The work of the Home Office is of fundamental importance to the very fabric of our society and I know better than anyone the dedication and hard work staff put in to make Britain safe and secure. Delivering a safe and secure Olympic Games is the most recent example, but there are many more.
"I will be doing my utmost over the next few weeks to ensure that the Home Secretary and her ministers have all the support they need in this period so that the department is in the best possible shape for the future."
Dame Helen, who had previously been permanent secretary at the Department for Environment Food and Rural Affairs, only took up her current post at the start of last year.
Home Secretary Theresa May paid tribute to her work.
"I am very grateful to Helen for her support, wise counsel and outstanding leadership of the Home Office during an extremely busy period for the department," she said.
"She has made a great contribution to the civil service throughout her long and distinguished career and I am sure she will continue to play a significant role in public life in her new role as director general of the National Trust."
Helen Kilpatrick, the director general of the financial and commercial group at the Home Office, will stand in as interim permanent secretary until a replacement is appointed.