Edward Leigh has called for changes to the Equality Act in order to prevent further cases similar to the man who was fired for declaring his opposition to equal marriage on Facebook.

Mr Leigh, Conservative MP for Gainsborough, said he fears that opponents of equal marriage who work in the public sector will be “treated as outcasts” unless the Equality Act is changed.

Mr Leigh proposes to amend the Equality Act by adding “conscientious beliefs about the definition of marriage” to the list of characteristics on grounds of which it is illegal to discriminate against people. The existing list of characteristics includes race, gender, sexual orientation and religion.

His concerns have followed in the wake of a letter to David Cameron sent by Aidan O’Neill QC, a lawyer and anti-equal marriage campaigner. Mr O’Neill warned that the government’s plans for equal marriage legislation could form a platform for suing the Church of England and other religious organisations, despite minister’s efforts to “quadruple-lock” them out of being required to perform same-sex marriages.

Mr Leigh said: “As Aidan O’Neill’s legal opinion suggests, if the government is successful in redefining marriage, then there are hundreds of thousands of teachers, parents, foster carers, or even hospital and army chaplains who could find themselves being disciplined for their beliefs, just as Adrian Smith was.”

Adrian Smith lost his managerial position and had his salary cut by 40% by Manchester’s Trafford Housing Trust after he commented in an online news discussion about gay couples marrying in church by saying it was “an equality too far” in February 2011.

Mr Smith brought breach of contract proceedings, saying the Trust acted unlawfully in demoting him. In November 2012 Mr Justice Briggs ruled in his favour at London’s High Court, however his previous position at the Trust could not be reinstated.

In order to prevent similar cases occurring again, Mr Leigh will call on MPs to support his amendment to the Equality Act by putting a ten minute rule bill before the House of Commons next week.

He said, “If the government is serious about protecting those who back the current definition of marriage from being marginalised for their beliefs then it must act immediately to change the Equality Act.

“If it does not then it will open the door to Christians, Muslims, Jews – and anyone else with a conscientious objection – being disciplined, demoted, or even sacked for backing the current definition of marriage.”

Mr Leigh cited records of the number of times the UK had been involved in European Court of Human Rights cases, stating that the UK was found to have breached international human rights law in 271 out of 443 cases.

He argued that it would be the “height of arrogance” to assume that UK law allowing religious organisations not to opt-in to conducting same-sex marriages would trump international law.

MPs are set to debate and vote on the government’s equal marriage plans for England and Wales before Valentine’s Day.