The Government’s plan to introduce same-sex marriage is one of the most serious threats to the Church of England in its 500-year history, senior clergy claim.

The Church today outlines its opposition to the Government’s proposals in scathing terms. Anxiety among Church leaders is so acute that they raise the spectre of disestablishment, warning that any attempt to alter the definition of marriage could fatally undermine the Church’s privileged position.

Ever since the reign of Henry VIII the Church of England has been the country’s official religion, facing down threats to its establishment as severe and varied as the Spanish Armada and the English Civil War. That senior clergy have raised concerns about same-sex marriage in a similar context indicates how seriously they view the Government's attempt to redefine marriage – as a potential attack on the role of the Church itself.

Critics have dismissed the Church’s stance as overly dramatic and called on bishops to follow the lead of established religious bodies in Iceland, Sweden and Denmark who largely embraced gay marriage.

The Church’s position, which was drawn up by senior bishops and lawyers, is confirmation that despite supporting civil partnerships eight years ago, the Church believes extending marriage rights to same sex couples is simply a step too far. The clerics say that the plans for same-sex marriage “have not been thought-through properly and are not legally sound”.

Downing Street has insisted that its plans to bring in equal marriage laws will go ahead. In March the Government launched a three-month consultation process calling on supporters and opponents to put forward their views with the deadline for submissions closing later this week.

The Church of England’s response lists a number of key reasons why they cannot support same sex marriages both theological and practical. At the heart of the debate is whether the definition of marriage can be changed from the lifelong union of a man and woman to that of any couple.

The government insists the change is a simple one which would allow “all couples, regardless of their gender, to have a civil marriage ceremony.” No religious organisation would be compelled to conduct a wedding ceremony as they would not take place in religious buildings.

But the Church counters that the proposals changes the very meaning of marriage, which is defined by both canon and parliamentary law.

Speaking to The Independent yesterday the Bishop of Leicester, Tim Stevens, criticised the speed with which the consultation on same sex marriage was being pushed through and added that “unintended consequences” could threaten the church’s historical role.

“If a category of marriage is created which separates the Church’s understanding of marriage from that of the state, it is bound to have some effect on the relationship of the church and its locality,” he said.

“That begins to raise questions about the nature of establishment as we’ve understood it.”

Lawyers acting for the Church have advised that the current proposals could leave it vulnerable to legal attacks precisely because it is the established religion that is tasked with officiating marriages to everyone in their parishes. There is particular concern that the European Court of Human Rights might force them to conduct gay wedding ceremonies if the meaning of marriage under British law was changed to include a couple regardless of their gender.

“It seems to me to be on the face of it at least possible, and perhaps more likely probable, that a challenge would be brought before the courts,” predicted Bishop Stevens. “And that it could be argued that for the established church not to make its premises available to people purely on the grounds of their sexuality could be regarded as discriminatory. The lawyers are arguing that it’s very likely that there’s a serious prospect that a successful challenge could be mounted in the courts.”

The Church’s stance will please its traditionalist and social conservative wings but will cause dismay among more liberal congregations who have campaigned to see it embrace equal marriage rights.

Symon Hill, from the Christian think-tank Ekklesia, commented: "The Church of England has missed an opportunity to move on from the defensiveness which has characterised many debates over same-sex marriage. This is particularly disappointing given that many of the Church of England's own members are far more positive about same-sex marriage than this official statement suggests.”

He added: “Marriage has been redefined many times throughout history. When married women were given the right to own property in 1882, there were those who argued that the new law undermined marriage. Similar claims were made when laws were passed to protect women from domestic violence and rape. Marriage has meant many things in many cultures.”