A new education Bill in Wales is threatening to scrap existing protections around sex education, the Christian Institute has warned.
The Curriculum and Assessment Bill was introduced to the Senedd over the summer by Minister for Education, Kirsty Williams, and is to have its first debate on Tuesday.
The Bill proposes wideranging changes to the teaching of sex education and religion in Wales, including new compulsory Relationships and Sex Education (RSE) lessons.
This would overturn the current system where primary schools can choose not to teach sex education, and where at secondary school level, parents have the right of withdrawal.
This is despite the Welsh Government consultation last year finding that 87.5% of people did not agree with making RSE compulsory for 3- to 16-year-olds.
Lessons in non-religious views like atheism and humanism would also become mandatory as part of a proposed new Religion Values and Ethics (RVE) curriculum to replace Religious Education.
The section on RVE states that teaching and learning "must reflect the fact that the religious traditions in Great Britain are mainly Christian, while taking account of the teaching and practices of the other principal religions represented in Great Britain", but it also goes beyond current law to state that they "must also reflect the fact that a range of non-religious philosophical convictions are held in Great Britain".
Simon Calvert, Deputy Director for Public Affairs at The Christian Institute, said the Bill risked "stripping away protections that took decades to achieve", and exposing children to "unsuitable materials that would not have been allowed under the previous law".
"Sex education groups have a long track record of pushing unsuitable materials on schools. Teachers are under huge time pressure and few specialise in sex ed so they could be dependent on these outside agencies for materials," he said.
Mr Calvert said that existing law offered "an important backstop against unsuitable teaching" that should not be removed, while RVE would impose a "one-way street that favours atheism over religion".
"With the rearrangement of the system for agreeing the RE syllabus, groups like Humanists UK, whose stated goal as recently as 2011 was to rid society of religion, are being given a veto on the content of RE. This is ridiculous," he said.
"Parents are not going to like being sidelined from their children's education on sex and religion – the two most controversial subjects you could imagine.
"And schools are not going to like being forced to teach controversial material that they, or the families they serve, disagree with. It could cause chaos."
In a statement responding to The Christian Institute's comments, Humanists UK said it was not out to rid society of religion and would not have any power of veto.
"The non-religious committee that is proposed to be added to each Local Authority Standing Advisory Councils on Religious Education (SACRE) will not have 'a veto power over all religious teaching'," it said.
"The proposal will merely allow representatives of non-religious groups, such as humanists, to participate in these bodies in the same way as religious representatives have always done. Religious representatives don't have such a veto and neither will non-religious representatives.
"We don't know on what basis the Christian Institute make their claim that Humanists UK wishes to 'rid society of religion' but that certainly has never been our view (in 2011 or at any other time).
"Indeed, we work closely with a wide variety of religious groups on the RE curriculum and are a founder member of the Religious Education Council of England and Wales."
CORRECTION: This article has been amended to clarify that groups on SACRE have a single vote each.