Transgender girls have been given until 6 September to leave the Guides.
The deadline set by Girlguiding follows the organisation's announcement in December that membership would be "restricted to girls and young women", which it described at the time as a "difficult decision".
It was acting after last year's Supreme Court ruling that sex meant biological sex in equality law, it said on Tuesday.
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Since that time, the umbrella organisation said it had undertaken "detailed considerations, expert legal advice and input from senior members, young members", its council and board of trustees.
The deadline gives affected members and their families "time to plan, prepare, access support, and decide when - between now and September - they feel ready to leave".
By that date, any trans girl or trans woman volunteering in a role open to women only, will need to move to a role that is open to all volunteers, or in other words, males or females.
Around 300,000 people aged between four and 18, are part of the organisation, members of the Rainbows, Brownies, Guides and Rangers groups within Girlguiding, as well as around 80,000 volunteers.
It also repeated its previous assertion that it doesn't know how many members and volunteers will be affected by the new gender rules as it doesn't collect gender identity information.
Girlguiding said it is still "committed to standing up for the rights, safety and dignity of girls and women, and to supporting marginalised communities, including LGBTQ+ people, to improve the lives of girls".
Campaign group Trans+ Solidarity Alliance described the news as "heartbreaking for the children and volunteer leaders that hold Girlguiding together".
It said: "Inclusive organisations being bullied into excluding people against their will is a profound failure of this government to live up to its promises to the trans community."
Following the initial announcement last year on the change to membership, a coalition of volunteers and parents argued that refusing a child based on their gender identity "sends a message of rejection" to young people.
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Guiders Against Trans Exclusion (GATE) coordinated demonstrations in various UK cities at the time.
But while some volunteers within Girlguiding have threatened to resign over the issue, others welcomed the change, saying they had expressed safeguarding concerns and were pleased at the new approach.
It was also announced in December that transgender women will be banned from becoming members of the Women's Institute from April.
(c) Sky News 2026: Transgender girls given until September to leave Guides
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