More than 1000 priests have signed a letter voicing concerns about how same-sex marriage will threaten religious freedoms and may even lead to Catholics being excluded from jobs.
In the letter, which is one of the biggest open letters of its type ever written, the priests claim that same-sex marriage could threaten freedoms in a way that was last seen during centuries of persecution of Catholics in England.
The letter, signed by 1054 priests as well as 13 bishops, abbots and other senior Catholic figures, expresses fears that the simple acts of practising or speaking about their faith will be severely limited.
They even claim the freedom to speak freely from the pulpit could be at risk.
Published in The Daily Telegraph, the letter goes as far as to compare David Cameron's proposed changes to the meaning of marriage to those of Henry VIII, whose efforts to divorce Catherine of Aragon sparked centuries of upheaval between Church and State.
Their fear is that Catholics who believe in the traditional meaning of marriage would effectively be excluded from some jobs - in the same way as Catholics were barred from many professions from the Reformation until the 19th century.
It said: 'After centuries of persecution, Catholics have, in recent times, been able to be members of the professions and participate fully in the life of this country.
'Legislation for same-sex marriage, should it be enacted, will have many legal consequences, severely restricting the ability of Catholics to teach the truth about marriage in their schools, charitable institutions or places of worship.
'It is meaningless to argue that Catholics and others may still teach their beliefs about marriage in schools and other arenas if they are also expected to uphold the opposite view at the same time.'
The Equal Marriage Bill, allowing couples of the same sex to marry, is due to be published this month.