The HIV and sexual health charity the Terrence Higgins Trust has asked Rosemary Gillespie, its chief executive, to stand down with immediate effect after 15 months in charge.
The charity said in a statement that its board had decided "a change of leadership is needed for the next stage of the organisation’s development".
A THT spokesman said Gillespie, who left the charity on Monday, had not received any additional payments for her departure other than those associated with her standard contractual notice period.
The charity was unable to confirm on Friday evening the length of Gillespie’s notice period. She is understood to have been on a salary of between £90,000 and £100,000 a year.
The trust employs 342 staff and had an income of £19.6m in the year to the end of March 2014.
A statement from Robert Glick, chair of THT, said it had been a privilege to work with Gillespie during a period of considerable change.
"She has been the driving force in developing a new strategy for THT, in making certain we have a solid organisational and financial footing for a more sustainable future and in helping to guide the vital work we do each and every day," he said.
"She leaves THT well positioned to become an even more effective organisation, poised to deliver on our dual ambition of stopping HIV transmission in our lifetime and in ensuring that people living with HIV can pursue healthy lives, free from prejudice and discrimination."
Glick said that one of Gillespie’s lasting legacies was to leave "the exceptionally gifted and skilled team of executives she has recruited and managed, who together lead our talented and dedicated staff".
He said the board would shortly be announcing the interim arrangements it will put in place while it recruits a permanent chief executive.
Gillespie joined THT in April 2014 from the HIV charity Avert, which had an income of £256,661 in the year to the end of March 2014 and employed 11 people.