Transgender women should be allowed to remain Freemasons if they joined as men, the largest UK lodge has said.
Expelling masons for transitioning was "unlawful discrimination", the United Grand Lodge of England (UGLE) warned.
Women who transition to become men should also be allowed to join, its new gender reassignment policy states.
The guidance document says that a Freemason's gender reassignment should be "treated with the utmost compassion and sensitivity".
Irrespective of gender identity, the UGLE's 200,000 members will still formally be referred to as "brothers", the document says.
But informally they should be addressed "by the name and title he/she has chosen".
It warns that using a mason's transition as a reason for excluding them from a men-only lodge would be "unlawful discrimination and so could never constitute sufficient cause".
The document issued to 7,000 lodges across the country, says: "If a Freemason who is a member of UGLE wishes to change gender and become a woman, we expect that the Freemason would receive the full support of their brethren."
Freemasonry, known in popular culture for arcane rituals and secret handshakes, raises millions of pounds for charity and has some six million members worldwide.
Famous Freemasons have included Sir Winston Churchill, Sir Arthur Conan Doyle, Rudyard Kipling and Oscar Wilde.
Each lodge holds four official meetings a year to welcome new members, and the ceremonies' contents have always been a closely-guarded secret.
Those now eligible to become a member of a male-only lodge include:
- A Freemason who joined as a man, but later underwent gender reassignment to become a woman
- A woman who underwent reassignment to become a man before applying to become a Freemason
While women cannot join the UGLE, there are two female-only lodges, the Order of Women Freemasons (OWF) and the Honourable Fraternity of Ancient Freemasons (HFAF), which also refer to members as "brothers".
There are about 4,700 female Freemasons in the UK.
The document says that it does not "impose binding rules" and "does not constitute legal advice".
But it adds: "A Freemason who after initiation ceases to be a man does not cease to be a Freemason.
"We expect that Freemasons will act with compassion and sensitivity towards their fellow Freemasons."
A spokesman for the UGLE said: "While there has been no general request for guidance on gender reassignment, questions on the subject are likely to become increasingly common in future, and now appears to be an opportune time to issue general guidance to our members."
In January the UGLE rejected a claim from the head of the Police Federation that Freemasons were blocking reforms and preventing women and minority groups from progressing
David Staples, the lodge's chief executive, said the suggestion was "laughable" and insisted Freemasons had a history of treating people "as equals".