Labour leadership contender Rebecca Long-Bailey has signed up to a pledge to expel party members who have expressed "transphobic" views.
It is part of a 12-point plan by the Labour Campaign for Trans Rights.
The pledge has now been signed by leadership contender Lisa Nandy too - but critics say it could lead to a "witch hunt" of party members.
It comes amid a highly-charged debate on gender identity including potential reforms to the Gender Recognition Act.
The recently-formed Labour Campaign for Trans Rights says its 12 pledges will help "rid the party of transphobia".
The pledges include accepting that there is "no material conflict between trans rights and women's rights" - and supporting the expulsion of Labour members who "express bigoted, transphobic views".
The group states that "trans women are women, trans men are men".
'Defend or expel us'
They also back the fight against what the group alleges are "transphobic organisations", naming two in particular; Woman's Place UK and the LGB Alliance.
Please sign to show your support for the trans and non-binary community, for whom the Labour Party should always be a safe space 🌹 https://t.co/qP7TjCEpA2— Rebecca Long-Bailey (@RLong_Bailey) February 11, 2020
Woman's Place UK, which says it campaigns to defend women's "hard-won rights", said it "absolutely refutes" claims their organisation is transphobic, describing the allegations as "scurrilous" and "defamatory".
Posting on Twitter, the group's co-founders said they were Labour members and called on the party to "defend us or expel us".
The LGB Alliance, who state that "biological sex is observed at birth and not assigned", promised to "keep speaking the truth and remain open to reasonable discussion".
A number of twitter users, who also say they are Labour members, are now using the hashtag #expelme in protest at the pledges, while others have suggested the trans rights pledge could lead to a "witch hunt".
In 2018, the government launched a consultation on reforms to the Gender Recognition Act, the results of which have yet to be published.
A Whitehall source has told the BBC the reforms are "on ice".
Leading LGBT charity Stonewall is among those groups who say that the current process, whereby trans people can have their identity legally recognised, is lengthy, intrusive and badly out of date.
But others have objected to what's known as self-identification or self-declaration, highlighting their concerns around the implications for women's sport and access to single sex spaces.
Broader arguments have played out in court, with the recent case of a woman who lost her job after saying that people cannot change their biological sex.
The woman, Maya Forstater, was among those to tweet the #expelme hashtag. She said she was a member of the Labour Party, and added: "If 'trans rights' means males in women's single sex spaces, sports & associations there is a conflict (with) women's rights."
Rebecca Long-Bailey, who is seen as one of the frontrunners in the Labour leadership contest, urged others to back the Labour Campaign for Trans Rights.
"Please sign to show your support for the trans and non-binary community, for whom the Labour Party should always be a safe space," she said on Twitter.
Angela Rayner, a contender for the deputy leadership, has also signed the pledge, saying: "Our solidarity is long overdue - The Labour Party should always be an open and safe space for all."
Later on Tuesday, Lisa Nandy's team revealed that she has also signed up to the Labour Campaign for Trans Rights' 12 point plan.
The leadership hopeful tweeted: "We've seen a rise in transphobic hate crimes in this country over recent years.
"Rather than allow women to be pitted against one another, the Labour Party should always be an open and safe space for all."
The Labour Campaign for Trans Rights is not an official Labour Party affiliate but, responding to the criticism on social media, a spokesperson said that transphobia is "totally antithetical to socialism" and that there is "clear evidence" that trans people are heavily discriminated against in society.
The other Labour leadership contenders, Sir Keir Starmer and Emily Thornberry, have been contacted for a response.