The news was announced by Nicholas Coleridge, the outgoing managing director of the magazine’s publisher Condé Nast Britain.
He said: "This is an announcement I hoped never to have to make. Alexandra Shulman, editor-in-chief of British Vogue for a quarter of a century, told me before Christmas that she had decided to stand down from Vogue in six months time.
"It is impossible to sufficiently express the contribution she has made to Vogue, to Condé Nast and to the British fashion industry."
Shulman is the longest serving editor in the 100 year history of the magazine, and has overseen its period of highest circulation. An announcement on her successor will be made in due course.
Shulman said: "I have edited British Vogue for 25 years almost to the day, and to have steered it during our spectacular centenary has been one of the greatest privileges.
"During that time I have worked with an unparalleled collection of talent both inside and outside the magazine and have been lucky enough to see both Vogue and the British fashion industry expand and flourish.
"It has been very hard to find a rational reason to leave what is unquestionably a fascinating and rewarding role but last autumn I realised that I very much wanted to experience a different life and look forward to a future separate to Vogue.
"My career at Condé Nast has been everything I could have wished of it and my heartfelt thanks go to Nicholas Coleridge and Jonathan Newhouse for giving me so many opportunities, trusting me to take care of the precious cargo that is Vogue and allowing me the freedom to do the job exactly how I wanted."
Coleridge will also exit his role at the publisher on 1 August. He will take the role of chairman of Condé Nast Britain until at least 31 December 2019.