The email arrived at 12.05pm. It contained the location of a feminist meeting to be held that night, less than seven hours later. “This information is confidential to ticket holders,” it warned.

Why the secrecy? This was the 23rd event in 20 months by A Woman’s Place, a loose collection of volunteers with roots in the trade union movement. The subjects under discussion can seem technical: reforms to the Equality Act, updates to census questions, the replacement of “sex” with “gender” in official forms. But the backlash has been fierce. Transgender activists have repeatedly tried to disrupt the meetings, describing them as hate speech. Venues have cancelled Woman’s Place bookings because of protests (and their associated security costs). An event in Oxford last year attracted condemnation from the student union, which accused the group of being “at the centre of this past year’s violent anti-transgender rhetoric and media abuse”.

There were no protests at the event in central London on 20 May, either because of the covert tactics or because the mood is changing. Gender-critical feminism is gaining ground, after decades in the wilderness. I could feel the relief of women who are used to Twitter beastings and whispered conversations, who were now able to see there are others like them. The audience was a Who’s Who of the current movement, known to each other from Mumsnet posts and Twitter pseudonyms. The audience was 95 per cent women, most in their 30s and upwards. The few men present included Father Ted writer Graham Linehan, who has become a Twitter warrior on this subject.

One of the event’s speakers, Julie Bindel, described the 15 years of ostracism that followed the publication of a piece in which she said that transgender ideology was grounded in sexist stereotypes. “Think about a world inhabited just by transsexuals,” she wrote. “It would look like the set of Grease.” After that, she was no-platformed by the National Union of Students in 2009 and repeatedly compared with Hitler. She noted dryly “that it’s never Pol Pot. None of the mid-range dictators ever get a look in.”

The audience at the Woman’s Place meeting were angry on her behalf, punctuating her speech with cheers and whoops. Bindel is one of the movement’s martyrs. Another is fellow speaker Maya Forstater, who has raised £60,000 on a crowdfunding site to take her employer to an industrial tribunal. She claims she lost her job for refusing to believe that “trans women are women”.

This line is the crux of the argument. The division between the feminists of A Woman’s Place and the “trans inclusive” branch of the movement is both simple and complicated. The first group see gender as a social force, imposed on women from outside: wear pink, do your hair, suck up all that unpaid caring labour. The second group believe in “gender identity” – an innate state of being. They use phrases such as “born in the wrong body”. Both positions have radical fringes. The extreme radical feminists believe that no amount of surgery, hormones or pronoun changes means a man should be treated by the law or society as a woman, or vice versa. Their mirror on the transgender side deny the existence of biological sex differences at all: to them, there’s no such thing as “male” and “female”.

The arguments about trans women are more bitter than the ones about trans men. That’s because the former are both more visible in the LGBT movement – trans men only seem to make the news when they have babies – and pose harder questions. In single-sex spaces that are segregated for safety reasons, such as changing rooms and prisons, should they be treated as men, women or a third category?

These are difficult and emotive questions, and they have created one of the deepest splits in modern feminism – and on the political left. A Woman’s Place draws its organising strength from socialist women, but many feel rejected by their own side. Meghan Murphy, a Canadian banned from Twitter for tweeting “men aren’t women”, said she felt deeply isolated in her own country. Bindel has ended up writing for the Daily Mail and Unherd as much as the Guardian. Earlier this year, activists from radical feminist group the Women’s Liberation Front (unwisely, in my view) sat on a panel hosted by the US conservative Heritage Foundation, which opposes LGBT rights.

With no home on the left, gender-critical feminism must resist allying with right-wingers who share none of its wider goals. A reckoning is also due with the movement’s own fringes, such as the handful of activists who heckled Debbie Hayton, a trans woman, when she spoke at a previous meeting. Hayton was there on Monday, wearing a T-shirt that read “transwoman: adult transsexual male”. Just as the pro-Palestine movement has to keep itself free of anti-Semitism, so the gender-critical movement needs a strong line to prevent activists sliding into transphobia.

The speeches mostly walked that line. Murphy referred to lesbians being asked to sleep with “heterosexual men interested in playing around with make-up”, but most of the digs came at the expense of young queer activists. Mentioning blue hair was a guaranteed applause line, as was Bindel’s reference to “wokeing class” men. The most stirring speech of the night was from Selina Todd, a historian of the working class, who made the case that queer theory and the transgender movement represented a move away from collectivist politics towards individualism. “Gender was not, and is not, an identity freely chosen,” she said.

The packed hall felt like the birth, or rebirth, of something. A feminism unafraid to talk about the female body. A rejection of the extremes of identity politics. And – just as radically – a movement that happens in the real world rather than purely online.