Opponents of gay marriage took the battle to schools today, with a letter to every state secondary school in England and Wales suggesting they will be forced to teach that gay and heterosexual relationships are equal.

The Society for the Protection of Unborn Children (Spuc) said even faith schools would be forced to teach the equality of gay marriage and that no individual teacher would be able to conscientiously object.

"The change to redefine marriage is something that's going to affect society in a large number of ways, not just the narrow question of a law on marriage," Spuc's Anthony Ozimic told politics.co.uk.

"Teachers would be forced to endorse gay marriage or face the sack.

"This is using equalities policies to force teachers to accept gay marriage and teach that it should be equal to true marriage."

The group used the letter to argue that section 403 of the Education Act 1996 - which forces state-maintained schools to teach "the nature of marriage and its importance for family life" - would automatically include gay marriage once the relevant legislation was passed.

"It is important to recognise that the equal marriage proposals will have a real, significant and disturbing impact on your school, and therefore it is essential that schools express their concerns about these proposals," the letter to headmasters reads.

The group insists that faith schools would not be exempt from the requirements, citing a comment from equalities minister Maria Miller last year in which she said they would have to "acknowledge that there can also be same-sex marriages".

Spuc also had legal advice published suggesting that if a teacher "refused to obey the otherwise lawful instructions of her employers then this would constitute grounds for her dismissal from employment".

The government is expected to soon press ahead with gay marriage legislation after a record-breaking public consultation on the issue.