The Roman Catholic Church has contacted every state-funded Catholic secondary school in England and Wales encouraging them to sign a petition opposing gay marriage.
The Catholic Education Service (CES) reportedly contacted 385 secondary schools asking them to circulate a letter read by two senior Archbishops during mass on 11 March which said their followers have a "duty" to ensure gays should not be allowed to marry in civil ceremonies.
Students at St Philomena's Catholic High School for Girls in Carshalton, south-west London, were shown a presentation on religious opposition to the government's plans to make gay marriages in civil ceremonies legal, Pink News reported.
It also asked pupils to "draw attention" to the Coalition for Marriage campaign and petition against civil marriage equality, a campaign which has had more than 446,000 to date.
One pupil at the school told Pink News she was "appalled and disgusted" at the presentation and said how children aged 11-18 were encouraged to sign the pledge by their headteacher.
"It was just a really out-dated, misjudged and heavily biased presentation" said the girl, who was not named in the report.
"Most importantly though, there are several people in my year who aren't heterosexual - myself included - and I for one was appalled and actually disgusted by what they were encouraging."
There are concerns from the British Humanist Association (BHA) that the CES's actions may have broken the Equality Act 2010 -which prohibits discrimination against pupils based on their sexual orientation - and 406 and 407 of the 1996 Education Act, which ban the political indoctrination of schoolchildren and require political views to be presented in a balanced way.
BHA Faith Schools Campaigner Richy Thompson said: "This action by the Catholic Education Service is absolutely outrageous. Not only might this break equalities legislation, it also breaks laws against political partisanship."
Terry Sanderson, President of the National Secular Society, said: "This is a clear breach of the authority and privilege that the Catholic Education Service has been given in schools.
"Surely it is no part of its remit to promote a specific political campaign from this purely sectarian viewpoint. It is disgraceful that children are being encouraged into bigotry when they are attending a state school paid for by taxpayers."
"The CES must withdraw this letter and undertake never again to use schools for ideological campaigning.
A spokeswoman for CES told the Guardian: "We said that schools might like to consider using this [letter] in assemblies or in class teaching. We said people might want to consider asking pupils and parents if they might want to sign the petition. It's really important that no school discriminates against any member of the school community.
"Schools with a religious character are allowed to teach sex and relationships - and conduct assemblies - in accordance with the religious views of the school. The Catholic view of marriage is not a political view; it's a religious view."
Greg Pope, the Deputy Director of the Catholic Education Service told PinkNews.co.uk: "We've asked them to draw attention generally to the Coalition for Marriage petition which is an open petition that people of all ages can sign.
"We have been aiming this towards older pupils and parents. The archbishop's letter is a positive statement of the Church's support for marriage, rather than negative comments about gay marriage."