Non-believers are not allowed to be full members of the conferences, although some committees have ignored that advice. Around 60 from a total of around 150 have some humanist representation.
A report by Asma Jahangir, the UN's special rapporteur on freedom of religion or belief, said it seemed vital that different views were properly represented when deciding the curriculum.
Her recommendations follow an announcement by the Government that it will review RE teaching after it was criticised by Ofsted for its inconsistent quality.
Andrew Copson, director of education at the British Humanist Association, said: "This report points out that the state has a responsibility to ensure that the content of syllabuses for RE is fair and we believe that can only be done through a statutory national curriculum."