Twitter is banning women who "speak out against the dangerous dogma of trans ideology", a feminist group has said.
In a letter to Twitter director Martha Lane Fox, Fair Play for Women says the company is allowing "a concerted attack on women's free speech".
But trans activist Ashleigh Talbot said the group's letter seeks to "whip up" hatred against trans people.
Twitter said its rules are enforced equally for every user, regardless of the commentary they engage in.
Fair Play for Women describes itself as a group of "ordinary women" who argue that "in the rush to reform transgender laws" women's voices will not be listened to.
It says Twitter users have been banned for stating "basic, incontrovertible biological facts" such as saying men are not women.
Their letter links to comments, that it says has led to people being banned from the site:
Transsexual writer Miranda Yardley said she was banned from Twitter for stating that Green Party LGBT spokesperson Aimee Challenor, a trans woman, is a man.
Writing on her blog she said: "According to the rules of Twitter it is now hateful conduct to call someone who is a man, a man.
"The implication of this is that the concept of proscribed speech, things we are now not allowed to say, now extends to the truth. This is fundamentally illiberal."
Fair Play for Women also said women receive abuse for talking "about their biology" - including being threatened with violence and referred to as Terfs (trans-exclusionary radical feminists).
The Green Party's Aimee Challenor said: "Miranda Yardley's comments about me are abusive and damaging. Twitter is right to take action against this form of online harassment which goes against Twitter's rules."
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