Parents from different faith backgrounds have united to commence a judicial review into the new relationships and sex education (RSE) curriculum.
The judicial review, launched by the Let Kids Be Kids Coalition (LKBKC), will challenge the Education Secretary's decision not to allow parents to withdraw their children from relationships lessons.
The mandatory curriculum came into effect in all schools across England on 1 September, and will include topics like transgender and LGBTQI issues.
While schools are required to consult with parents on the content of the RSE lessons, parents do not have any right of veto.
Critics of RSE say the relationships lessons include inappropriate material that should be restricted to sex education lessons - where parents still have a right of withdrawal up to their child's 15th birthday.
LKBKC voiced concern that books on sexual orientation and gender dysphoria, like 'King and King', 'And Tango Makes Three' and 'My Princess Boy', are "becoming normalised" in primary schools.
The coalition's chair Charlie Colchester previously told Christian Today that relationships education had become "sex education by a different name".
LKBKC, which has united Christians, Jews, Muslims and other faith groups, says children should not be exposed to ideologies "which do not reflect the religious or philosophical convictions of families".
Government guidance to schools on the implementation of the curriculum states: "LGBT teaching should be fully integrated into the school; and not taught as a stand- alone subject."
LKBKC is crowdfunding to cover the costs of the legal challenge, which is seeking to stop the guidance from being implemented, and make RSE classes optional.
Commenting on the decision to seek a judicial review, Mr Colchester said: "There is ideological pressure to sexualise children at an early age. The Government, in thrall to propagandists, is steamrollering parents' age-old rights. Why?
"What have parents done wrong, that they are being targeted in this way? Parents have the primary responsibility for the upbringing of children; children are vulnerable and transgender and LGBT teaching to primary school children is age-inappropriate and will only confuse them."
In a letter to the Government ahead of the judicial review, the coalition warns that England has been left with "an educational regime which, taken as a whole, no longer gives effect to the rights of parents to ensure teaching in conformity with their own religious and philosophical convictions, and is therefore in breach of the parents' rights".
The Government rejects the assertion that it is in breach of the law, and has confirmed that it intends to fully implement the curriculum now that schools have returned from lockdown.
Mr Colchester said parents had been left "powerless" by the guidance.
"In 2011 a Conservative Prime Minister, David Cameron, campaigned against 'the oversexualisation of children'. In 2020, a Conservative administration has declared 'open season' on the sexualisation of children. Why? What on earth have children done to deserve this fate?" he said.
"The removal of the right to opt-out of classes without explanation is a fundamental change to the 'right to withdraw' that parents have had for decades in relation to sex education. Now parents are powerless."