The Church of England has told MPs that it cannot support the legalisation of gay marriage because of the "uncertain and unforeseen consequences for wider society and the common good".
Concerns were set out in a briefing on the Marriage (Same Sex Couples) Bill sent to MPs prior to the second reading debate in the House of Commons on 5 February.
The Church said the Government's plans were an "unnecessary politicising" of marriage and had "no mandate".
In the event that the bill does become law, safeguards for churches and other faith groups would be "essential", the Church said, as it expressed "continuing anxieties" about whether protections for the religious freedom of Christians outlined in the current draft of the bill would be adequate.
The Church reiterated its support for civil partnerships but said that applying uniformity to different relationships would be "an unwise way of promoting LGBT equality".
The briefing also raised concerns about the religious freedom of Christians and others with a traditional view of marriage.
"The continuing uncertainty about teachers, the position of others holding traditional views of marriage working in public service delivery, and the risk of challenges to churches in the European courts despite the protections provided, suggest that if the legislation becomes law it will be the focus for a series of continued legal disputes for years to come," the Church said.
"Whilst some fears about freedom of expression may have been exaggerated, we doubt the ability of the government to make the legislation watertight against challenge in the European courts or against a 'chilling effect' in public discourse.
"We retain serious doubts about whether the proffered legal protection for churches and faiths from discrimination claims would prove durable.
"Too much emphasis, we believe, is being placed on the personal assurances of ministers."