Concern has been raised over biological males sharing accommodation, showers and toilets with girls on trips away since the policy change.
Ms Alcock’s dispute with the organisation started in June 2018 when her objections to the transgender policy were secretly reported to the Girlguiding leadership.
A four-month investigation concluded she had breached the organisation’s social media rules by airing her concerns online.
It also ruled she had broken the ‘code of conduct’ by refusing to adhere to the Guides’ equality and diversity policy on transgender inclusion.
She was then expelled when her membership of Girlguiding was revoked after she refused to agree to follow the new transgender rules.
She appealed against her expulsion, but the original decision against her was upheld.
In April she lodged a civil case against Girlguiding in London County Court.
Her solicitor Peter Daly, of Slater And Gordon lawyers, said: “If Katie is successful, it may establish legal protections on which other people who share her beliefs can rely.”
A Girlguiding spokesman said: “We are unable to comment on an ongoing legal case.”