Martina Navratilova will be the mystery guest at next week’s Stonewall fundraising dinner.

The tennis legend, who revealed this week she is being treated for breast cancer, will make the keynote address at the £175-a-head event at the Dorchester hotel in London.

Chief executive of the gay charity Ben Summerskill told the Daily Mail: “Despite everything, she is still coming here and needless to say we are absolutely delighted. She is a terrific person and a terrific ambassador for gay rights.”

Navratilova came out in 1981 and has been candid about being a lesbian ever since.

She follows figures such as Tony Blair, Stephen Fry and Ian McKellen in addressing the yearly event, although she is not a professional public speaker.

Earlier this week, the nine-times Wimbledon champion said that the day she got her cancer diagnosis was her “personal 9/11″.

She has since had a lumpectomy and will begin a six-week course of radiation therapy next month in Paris, where she will also work as a commentator for the French Open.