A group of concerned parents has been granted a judicial review into the new Relationships and Sexuality Education (RSE) curriculum being introduced to Welsh schools this year.
Mandatory RSE lessons for children aged three and up will be rolled out across Welsh schools this September.
Subjects covered in the lessons include relationships, sex, gender, sexuality and sexual health.
Welsh government guidance states that the lessons must be appropriate to the age, knowledge and maturity of learners.
Parents do not have the option of withdrawing their child from these lessons.
A campaign backed by over 5,000 people expresses fears that very young children will be sexualised and introduced to "sensitive and inappropriate topics such as gender ideology".
The campaign also claims that parents will be "disenfranchised by being denied their time-honoured right to remove their child from sex education".
Mr Justice Turner, sitting at the Royal Courts of Justice, has granted the request for a judicial review which must now be heard before the start of the September term.
The parents will be represented by leading human rights barrister Paul Diamond.
Granting the judicial review, Justice Turner said: "The issues raised on behalf of the claimants involve the consideration of complex constitutional matters with potentially very significant consequences for both parents and children.
"The response of the defendants, although not without force, is not so obviously determinative of the issues as to justify the refusal of permission."
The campaign against mandatory RSE is being led by Public Child Protection Wales which has grown increasingly alarmed about what children are being taught in schools.
It has heard reports of female teenagers in one school being encouraged to "explore" whether they are transgender because they prefer boys' games and activities. In another school, vibrators were reportedly handed out to a mixed class of 12 year olds, with students asked to practise rolling down condoms on them, leaving many of the girls and some boys traumatised.
The ruling by Justice Turner has been welcomed by Kim Isherwood, a founding member of Public Child Protection Wales.
She said: "After years of being ignored, we are delighted that the judge has recognised the seriousness of the proposed changes, the withdraw of time-honoured parental rights and, that the RSE programme, as it stands, could have significant consequences on children's mental and physical health.
"In one swoop, the Government has broken the vital partnership between parents and schools by, for the first time, creating a political wedge between us. Children should not be used for political ideological experiments in relation to identity and sexuality.
"It is tantamount to institutionalised abuse, asked to be delivered by teachers who choose a vocation to teach every child as an individual, with respect and dignity, and not to force any ideology on them.
"School is a place to learn about vital biology, how to develop relationships (with both sexes) and to develop respect.
"But, as parents, we believe what is happening here is the sexualisation of children, not the education of children. This is a social engineering experiment which must be stopped."
