Tampon brand Tampax faced a backlash yesterday after claiming "not all people with periods are women".
In a tweet championing transgender rights, the firm also insisted "not all women have periods", adding: "Let's celebrate the diversity of all people who bleed!"
But some feminists branded the post "virtue signalling" and accused Tampax's owner, Proctor & Gamble, of "erasing" female identity.
Writer and feminist Susan Dalgety, who backed author JK Rowling amid a transgender row earlier this year, said: "It's a biological fact you need a uterus to menstruate, and that only females have one. Males do not.
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"I have no idea why Proctor & Gamble thought it was a good marketing tactic to alienate their sole customer base in this way."
Kellie-Jay Keen-Minshull, founder of the Standing For Women campaign group, said: "They are virtue signalling. It's quite frightening. When you try and include all identities into womanhood, you ultimately exclude women."
It comes after high street chain Superdrug last month unveiled a new range of sanitary products "for people who menstruate".
In June, JK Rowling was accused of being "transphobic" after insisting only women experience menstruation. She had challenged an article entitled: "Opinion: Creating a more equal post-Covid-19 world for people who menstruate.
Proctor & Gamble did not respond to requests for comment last night.