A catholic school in Carshalton is at the centre of a row over schoolchildren being encouraged to join a campaign against gay marriage.

The Roman Catholic Church has written to every state-funded Catholic secondary school in England and Wales asking them to make pupils aware of a petition against gay marriage.

A student at St Philomena’s Catholic High School, in Carshalton, has said girls had been “encouraged” to sign the anti-equality pledge by the school’s headmistress.

Youngsters at the school were shown a presentation on religious opposition to the government’s proposals to allow gay couples to marry in civil ceremonies.

Some students have responded by buying Gay Pride badges to pin to their uniforms.

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."

A statement from the school said those under the age of 16 were informed they could not sign the petition which proposes the legal definition of marriage stays the same.

It said the assembly focused on the Pastoral Letter of The Bishops Conference on the Church’s teaching on marriage and was delivered in response to a request by the Catholic Education Service.

It said: “As a Catholic school, we have a duty to inform our students of the Church's teaching on social issues while also promoting, supporting and respecting pupils' right to think for themselves.

“St Philomena's is committed to providing a welcoming, equal and inclusive environment for all our students. We make this clear in our equality and diversity information which is on the school's website.

“St Philomena's continues to provide education and pastoral care to all our students, in accordance with the principles of equality and diversity.”