A Christian family is suing their son's Church of England primary school after a boy was allowed to wear a dress to class.
Nigel and Sally Rowe are threatening legal action after pulling their six-year-old son out of school.
They removed their other son, now aged 8, from the same school a year ago when a boy in his class started wearing dresses.
The couple plan to challenge the school's uniform policy and will argue that by allowing boys to wear dresses the school has not respected parents' right to bring up their children with biblical values.
"A child aged six would sometimes come to school as a girl or sometimes come to school as a boy. Our concerns were raised when our son came back home from school saying he was confused as to why and how a boy was now a girl," Nigel Rowe, 44, told The Sunday Times.
"We believe it is wrong to encourage very young children to embrace transgenderism. Boys are boys and girls are girls. Gender dysphoria is something we as Christians need to address with love and compassion, but not in the sphere of a primary school environment," he added.
The boy who wore a dress to school also turned up at the Rowe son's royal-themed birthday party in a blue velvet dress.
The school, which has not been named publicly, said it had policies in place to tackle transphobic behaviour.
It said it was following Church of England guidance in allowing children to choose whether to wear dresses or trousers. Transgender children are protected under the 2010 Equality Act, the school said, adding that East Sussex county council's guidance states that they should not be viewed as a problem, but "as an opportunity to enrich the school community and to challenge gender stereotypes and norms on a wider scale."
Lawyers representing the Rowe parents are likely to argue that the school has discriminated against the couple by implying that their wish to bring up their children with biblical beliefs is transphobic.
The Rowes are being supported by the Christian Legal Centre. Andrea Williams, the centre's chief executive, commented: "This new transgender ideology is being aggressively imposed on unsuspecting schools, parents and children. It is delusional and abusive. School classrooms, which should be one of the safest environments for children, are rapidly becoming dangerous battlefields in a war brought on by a radical transgender ideology. Vulnerable children are being used as pawns and will be harmed the most. We need to call it what it is."