Sir Nick Partridge, the chief executive of the Terrence Higgins Trust, will step down in October after almost 30 years at the charity.
Partridge was the charity’s first paid member of staff when it began life in a small London office in 1985 and he went on to become its chief executive in 1991.
The charity said it is now the largest HIV and sexual health charity in Europe, with 30 centres in the UK and an annual turnover of £20m.
Paul Ward, the trust’s current deputy chief executive, will become acting chief executive from November but the charity has decided to combine the two jobs to reduce its costs and plans to advertise the new role soon.
Partridge said: "It’s been an extraordinary three decades, and a privilege to serve such a remarkable charity as chief executive. Having led Terrence Higgins Trust through it’s 30th anniversary, the time feels right for me to move on to other challenges."
Partridge, 58, now plans to concentrate on his other roles as deputy chairman of the UK Clinical Research Collaboration and as a non-executive director of the Health and Social Care Information Centre.
Professor Christopher Bones, chair of trustees, said: "We are hugely grateful to Sir Nick for his enormous contribution to the Terrence Higgins Trust over the past three decades. His vision, networks and insight are huge assets, and I am delighted that he will remain an important part of our family in the years to come."