The Roman Catholic church has written to every state-funded Catholic secondary school in England and Wales asking them to encourage pupils to sign a petition against gay marriage.
Students at one south London school were shown a presentation on religious opposition to government plans to let gay couples marry in civil ceremonies. Church leaders believe the proposal would reduce the significance of marriage.
The Catholic Education Service, which acts for Catholic bishops in England and Wales, contacted 385 secondary schools to highlight a letter read in parish churches last month, in which two archbishops told worshippers that Catholics have a "duty to do all we can to ensure that the true meaning of marriage is not lost for future generations".
The CES also asked schools to draw pupils' attention to the petition being organised by the Coalition for Marriage, a Christian campaign which has attracted more than 466,000 signatures to date.
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.