David Cameron is under renewed pressure over Government plans to introduce same-sex marriage as the Church of England and the Roman Catholic bishops of England and Wales launched scathing criticisms of the proposals.
The Church of England accused the Government of failing to think through its plans to introduce same-sex marriage, adding that the legislation would undermine the Church's relationship with the nation and be subject to a legal challenge from Europe.
Introducing gay marriage would lead to an unprecedented clash between its own canon law - that marriage is between a man and a woman - and that of Parliament, the Church of England warned.
It added that in spite of ministerial assurances that churches would not have to conduct gay marriages, it would be "very doubtful" whether limiting same-sex couples to non-religious ceremonies would withstand a challenge at the European Court of Human Rights (ECHR).
A successful legal challenge could make it impossible for the Church of England to continue its role conducting marriages on behalf of the state, it said.
The strength of the criticism prompted gay rights campaigning group Stonewall to dismiss the claims as a "masterclass in melodramatic scaremongering" by the Church of England.
The Bishop of Sheffield, the Rt Rev Steven Croft, said the Government proposals represented a "fundamental change" to a "very, very important" social institution.
He said: "Whilst this is being presented as a kind of minor extension to what marriage means, actually, from the point of view of the Church and of society, it is a really, really fundamental change to an institution which has been at the core of our society for hundreds of years and which for the Church is not a matter of social convention but of Christian doctrine and teaching.
"One in four marriages in England are performed by the Church of England and that proportion is rising at the moment.
"In every marriage service the priest begins the service by spelling out what marriage is - a union between one man and one woman with the intention of it being lifelong.
"So it is really important to register back to the Government that this is not a minor change, this is a fundamental change to a very, very important social institution."
But Ben Summerskill, chief executive of Stonewall, dismissed fears raised by the Church of England that introducing same-sex marriage would undermine its centuries-old role as the established Church.
He said polling commissioned by Stonewall has shown more than 80% of people in Britain under 50 are in favour of introducing same-sex marriage - among religious people, he said, three in five said gay people should be able to get married.
"That holes below the water-line the tendentious claims being made by a small number of clergy," he told BBC Radio 4's Today programme.
"Many bishops in the Church of England today will be rather pleased because once again they are not talking about global poverty or the HIV pandemic - they are talking about the subject that obsesses them, and that is sex.
"I have not come across such a masterclass in melodramatic scaremongering - that somehow this is the biggest upheaval since the sacking of the monasteries - since as a journalist myself a decade ago I was summoned to a government briefing to be told about weapons of mass destruction in Iraq."
The Roman Catholic Church in England and Wales, in a separate submission to the Government consultation on same-sex marriage, also accused the Government of putting forward plans which were not subject to "careful study and analysis" which would if implemented "permanently diminish" the significance of marriage.
A Downing Street spokeswoman insisted that the proposals would not require Church of England ministers to conduct same-sex marriages in churches.
She said: "The purpose of the consultation process is to enable us to listen to all views, including those of all religions.
"It is the Government's view that marriage is one of the most important institutions we have got.
"The consultation paper makes very clear that no religious organisation will be forced to conduct same-sex marriages as a result of our proposals.
"In terms of the Church of England's comments today, we welcome their response and we will carefully consider their points of view before we go ahead and publish the outcome of the consultation which will happen later in the year.
"The coalition parties have made clear we are committed to legislate by 2015 - in this Parliament. That is the Government policy."
The submissions come as a 500,000-strong petition from the Coalition for Marriage (C4M) opposing same-sex marriage was presented at Downing Street.
Mr Cameron, who has spoken out in favour of gay marriage, has come under fire from supporters of the proposals for allowing a free vote among Tory MPs to avoid a rebellion over the issue.
But other Tory critics have said they view the proposal as a Liberal Democrat policy distracting the Government from bigger challenges.