The Catholic Church has been accused of urging pupils as young as 11 to sign an anti-gay marriage petition.
The Catholic Education Service (CES), which promotes Catholic education, confirmed it wrote to 359 state schools in England and Wales to remind them of the traditional definition of marriage.
The CES was by the Catholic Church to forward the text written by Archbishops Vincent Nichols and Peter Smith which was read out during masses in March.
The message from the Church encouraged congregations to continue supporting the "true meaning of marriage" in the face of the government's consultation on legalising gay marriage.
According to a spokeswoman from the CES, the letter was distributed to schools - which were allowed to approach the letter in a way they thought was "appropriate".
"We asked schools to draw pupils' attention to the petition, which simply affirms Catholic beliefs," she told The Huffington Post UK.
But according to a student at St Philomena's Catholic High School in Carshalton, Surrey, pupils were shown an anti-gay marriage presentation on the campaign which ended with "Sign the petition".
"It was just a really out-dated, misjudged and heavily biased presentation," the sixth-former told Pink News.
Chief executive of gay-rights campaigners Stonewall Ben Summerskill said: "This is a shocking breach of the school’s duty of care. It categorically shouldn’t be involved in such a live political issue, particularly in a way that demeans gay pupils. We’re sure students and parents will be upset that this school is creating deeply unpleasant environment for gay young people when so many other faith schools are doing a brilliant job tackling homophobia."
The British Humanist Association have dubbed the incident "outrageous".
"Not only might this break equalities legislation, it also breaks laws against political partisanship."
CES, which acts as a liason between the government and Catholic bishops, insisted the move was "purely religious and in no way political".
The CES spokeswoman added the BHA was "confused".
"They've got themselves into a muddle," she continued. "It is a very well established law that Catholic schools are allowed to teach Catholic beliefs. They have always been allowed to teach sex and relationships according to the Catholic religion."
"It is important children are taught about Catholic values. We are supporting marriage as it is currently defined. It is a positive affirmation, not negative."
The CES emphasised the St Philomena's was responsible for designing the presentation. Asked whether the "sign the petition" slogan on the presentation impeded free will, the spokeswoman replied: "It is not as dogmatic as people are suggesting".
"Children are in schools to have their minds opened to different things and to learn.
"Nobody was forced to sign the petition. It was just suggested to them."
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