David Cameron has been sent a letter containing legal advice warning that churches could be sued under human rights laws, if they refused to perform equal marriages.

According to the warnings by leading human rights lawyer, Aidan O’Neill, plans to allow the Church of England to be exempt from marriage equalities by the government are “eminently challenge-able.”

The warnings from the Matrix Chambers lawyer, argues that protections only allowing religious groups to marry people if they opt-in, as laid out in the plan including a “quadruple lock” to protect religious freedom, could be undermined by evolving European human-rights law.

He went on to say that churches who refuse to perform equal marriages could be banned from using council facilities such as village halls.

Details of the warning from O’Neill were contained in a letter sent by Lord Carey, the former Archbishop of Canterbury, to the Prime Minister, reported the Daily Mail.

Opponents to equal marriage have commissioned other opinions, arguing that teachers could face disciplinary measures under equality laws if they refuse to teach lessons including the history of marriage equality, once the change is made.

Lord Carey warned the plans could lead to “serious and wide-ranging conflict between religious institutions and local authorities”.

He also argued that the legal opinion “demonstrates why the legislation is unworkable”. He concluded: “These proposals are divisive, have no mandate and are poorly thought-out.”

A Downing Street official said that the government “has legal advice of our own”, and that the Prime Minster would “press ahead” with the plans.

“It is precisely because the Church of England has a legal duty to marry that we have created a legal lock to ensure this doesn’t apply to same-sex couples,” a Government source said.

Same-sex couples in England and Wales are set to be allowed to marry, under plans announced by Culture Secretary Maria Miller in the House of Commons in December. The bill should be unveiled at the end of this month.

Mrs Miller told the House of Commons that she was putting in place a “quadruple lock” of measures to guarantee religious organisations would not have to marry gay couples against their wishes, including a ban on the Church of England, and Church in Wales performing same-sex weddings.

Speaking in favour of equal marriage, on which he has said he would like Tory MPs to have a free vote, David Cameron said he did not want gay couples to be excluded from a “great institution”.

Last week, more than a thousand Catholic priests signed a letter saying that if equal marriage is legalised it could threaten religious freedom in a way last seen during “centuries of persecution” of Catholics in Britain. 

Conservative MPs, including ministers, will get a free vote on the issue of equal marriage. The Labour Party has also granted its backbenchers a free vote.

PinkNews understands that MPs will debate and vote on the government’s equal marriage plans for England and Wales before Valentine’s Day. It is expected to take up to a year to pass through Parliament.