Trans women will not be able to participate in the main session of Labour's Women's Conference next year.
This decision follows a legal review following the Supreme Court's April ruling that a woman is defined by biological sex under the Equality Act.
Trans women will not be able to participate in formal proceedings, including speeches and policy debates in the main hall, but will be able to attend fringe events, which will be open to all regardless of their sex.
It is understood that the party considers this format the least restrictive way to balance accessibility and compliance with the law.
The Women's Conference, which usually takes place at the same venue the day before the party's annual conference, was postponed to 2025 after the party received legal advice following the Supreme Court ruling.
Before that decision, Labour allowed people who identified as women to attend the event and participate in "positive action" measures such as an all-women shortlist.
A spokesperson said that following the Supreme Court decision, Labour launched a "comprehensive legal review" of its women's conference rules, and then confirmed that the 2026 event would take place with new attendance rules.
The spokesperson added: "This reflects our commitment to addressing the underrepresentation of women in the party and complying with the law."
This comes as the government is considering a new code of practice in light of the decision, issuing guidance to public bodies and businesses on how to implement the Equality Act.
The code has been redrafted by the equality watchdog, the Equality and Human Rights Commission, and submitted to the government for ministerial approval before being implemented.
Equalities Minister Bridget Phillipson received the draft three months ago, but said she would take the necessary time to "correct it" before publishing it.
The EHRC, under its current leadership, had previously urged the government to approve the guidance quickly.
Outgoing EHRC chief Baroness Falconer of Margravine told The Times that she was "absolutely certain" about the validity of the guidance and suggested that one reason for the delay could be that the government was "afraid of its own MPs who want trans self-identification or trans inclusion to permeate all areas of society." She said this delay had created a "grey zone" around single-sex spaces.
Baroness Falconer said, "Some organisations are implementing it [the ruling] one way, others not."
The Labour Party's announcement comes after two high-profile organisations issued new regulations following the Supreme Court ruling.
Earlier this week, the Women's Institute announced that it would no longer grant membership to transgender women.
A day earlier, the UK Girlguiding organization said that trans girls would no longer be allowed to join Girl Guide groups.