Men who believe they are women will be allowed to become Guide leaders, under a new Equality and Diversity policy sent to Guide leaders this week.
Andrea Williams, Chief Executive of Christian Concern, described the move as a "dangerous capitulation to political correctness".
'Self-identify'
Girl Guides was founded in 1910 and, like boy Scouts, has always been strictly single-sex.
But in 2015, Girlguiding UK published gender guidelines which states that members be accepted on the gender they "self-identify" with.
This means that a boy is able to join the group simply because he 'feels' that he is a girl, even without taking hormones or other medical procedures.
The new document states: "Girlguiding aims to support all girls and young women. This is understood in terms of the way a person self-identifies their gender identity - a person's inner sense of self."
Parents not informed
Under the new rules, parents will not have to be informed if a Guide leader is a man identifying as a woman or if there are boys in members' groups.
According to the document, it is not seen as "best practice" to inform the parents, even with young girls sharing sleeping and washing facilities with a biological boy on residential trips.
Chief Executive Julie Bentley, who describes Guides as the "ultimate feminist organisation", said: "Girlguiding complies with the Equality Act 2010 which makes clear organisations providing single-gender services should treat people according to their acquired gender.
"In line with our values of inclusion, we welcome any young person who self-identifies as a girl or young woman."
'Shocking'
But the plans have been met with strong criticism from MPs and parents.
Amanda Gracey, a parent whose six-year-old daughter recently joined Rainbows, said it is "shocking that if there is a man who believes he is a woman leading the group, it is forbidden that parents should be informed, even on residential trips.
"You might be in a situation with girls having to deal with menstruation going to a leader with no understanding at all. Every piece of safeguarding advice says you should provide separate sleeping and changing facilities for children of opposite sexes under the age of 18."
Conservative MP Philip Hollobone said: "Sadly the result will be fewer girls joining the Girl Guides. There will be parents who won't let their daughters join because of this change."
'Dangerous capitulation to political correctness'
Andrea Williams, Chief Executive of Christian Concern, commented: "This is a dangerous capitulation to political correctness. The safety of young children may be compromised as a result of wishing to appear 'inclusive', and young boys who are experiencing confusion about their gender will only become more confused as they are placed in an all-female environment.
"Parents who have children attending guide groups need to stand up and speak out against these new guidelines."
Related links:
Boys who think they are girls allowed to join Guides for the first time (Telegraph)
Now boys can be girl Guides if they think they're the wrong gender (Mail)