More than 1,000 priests have signed a letter voicing alarm that same-sex marriage could threaten religious freedom in a way last seen during "centuries of persecution" of Roman Catholics in England.
In one of the biggest joint letters of its type ever written, they raise fears that their freedom to practise and speak about their faith will be "severely" limited and dismiss reassurances from the British government as "meaningless".
They even liken British Prime Minister David Cameron's moves to redefine marriage to those of King Henry VIII, whose efforts to secure a divorce from Catherine of Aragon in the 16th Century triggered centuries of bloody upheaval between English church and state.
They claim that, taken in combination with equality laws and other legal restraints, the coalition's plans will prevent Catholics and other Christians who work in schools, charities and other public bodies speaking freely about their beliefs on marriage.
And they fear that Christians who believe in the traditional meaning of marriage would effectively be excluded from some jobs – just as Catholics were barred from many professions from the Reformation until the 19th Century.
The comments are contained in a letter, signed by 1,054 priests as well as 13 bishops, abbots and other senior Catholic figures.
They account for almost a quarter of all Catholic priests in England and Wales.
It comes as opponents of gay marriage launch a lobbying campaign targeting MPs in 65 of the most marginal seats. The coalition is due to publish its Equal Marriage Bill, allowing couples of the same sex to wed at the end of this month.
Legal opinions commissioned by opponents have argued that teachers could face disciplinary measures under equality laws if they refuse to promote same-sex marriage.
The priests write: "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."
Arguing that marriage as traditionally understood is "the foundation and basic building block of our society", they add: "We urge members of parliament not to be afraid to reject this legislation now that its consequences are more clear."
A Department for Education spokeswoman said: "The government's proposals for equal marriage do not change anything about teaching in schools." (© Daily Telegraph, London)