Siân Berry has announced she will be leaving the Green Party’s leadership this autumn over divisions in the party about trans rights.

Berry, who has been the party’s co-leader in England and Wales since 2018, published a letter on Twitter on Wednesday, addressed to party members, saying that she will not stand as a candidate in its upcoming leadership by-election.

“There is now an inconsistency between the sincere promise to fight for trans rights and inclusion in my work and the message sent by the party’s choice of front bench representatives,” she explained in the letter, which said she felt “sorrow” over this.

“We must all ask ourselves important questions about the values our party upholds,” Berry added. “Will we continue to embrace the principles of listening and solidarity when minority groups are singled out for attack?”

“More urgently than ever,” she argued, “we need a transformed society that brings us together, respects everyone’s rights and leaves no gaps for people to fall through, and a transformed economy that respects environmental limits.”

In December, Kathryn Bristow, a trans woman, was elected co-chair of Green Party Women – a subgroup that represents the priorities of women party members – prompting what Berry called a “very upsetting” backlash from groups that oppose trans rights and “a very small number of people” in the party. In an official statement at the time, the party said its support for trans rights was “unequivocal”.

During her unsuccessful campaign for London mayor earlier this year, Berry vowed to make the capital the “most trans-inclusive city in the world”. As part of this, she pledged, if elected, to set up a commission to examine the needs of trans Londoners and to plan how to tackle discrimination in healthcare, employment and housing.

Earlier this month, Berry’s long-term co-leader Jonathan Bartley also announced his plans to step down. Like Berry, he had supported trans rights publicly, though did not mention divisions over this issue in the context of his resignation.

‘A brave and principled decision’

In response to Berry’s announcement, Martin Phipps, a Green Party councillor in Sheffield, thanked her on Twitter for her “strong stance in support of trans rights”, adding: “Green parties are, and must be, rainbow parties.”

Alex Charilaou, an organiser at the NGO Progressive International, said: “Thank you for being one of the only UK party leaders with principles on trans rights.”

Andrew Boff, the Conservative chair of the London Assembly, called Berry’s announcement: “A brave and principled decision. We have many differences, but where we are united is asserting that #TransRightsAreHumanRights.”

The UK has become notorious among international human rights observers for rising opposition to trans rights, including from some feminists.

In May, Jeff Ingold, the head of media at Stonewall in the UK – which calls itself the largest LGBTIQ rights charity across Europe – announced that he plans to take a year-long sabbatical amid “a tsunami of transphobia”.

Levan Berianidze, former head of the Equality Movement, the largest LGBTIQ rights group in conservative Georgia (Eurasia), warned in an article for openDemocracy that, given the UK’s global influence, its divisions over trans rights also “threatens to undermine rights in less privileged parts of the world”.