A former minister is pressing for a change to the law to protect people who believe in a traditional definition of marriage.
Tory MP Edward Leigh will bring forward a bill in the House of Commons before the end of the month seeking urgent changes to the Equality Act.
He warns that, without legal protection, thousands of people could be "treated as outcasts" for their views, while in the workplace, employees could be disciplined or sacked.
Mr Leigh is calling for change in response to the recent case of Adrian Smith, who was disciplined by Trafford Housing Trust for posting a message on his personal Facebook page in which he said that gay marriage was "an equality too far".
Mr Smith was demoted by the trust from his managerial position and had his pay cut by 40 per cent. He eventually won a legal challenge against the trust at the High Court but was not reinstated to his former position.
Mr Leigh said he shared the concerns of human rights lawyer Aidan O'Neill, who recently warned that, without adequate legal protections, there would be serious consequences for those with traditional views on marriage.
"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. To think otherwise is out of touch with reality," he said.
"If it does not then it will opens 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."
The Government has promised a "quadruple lock" on churches and clergy who do not wish to perform same-sex ceremonies.
Mr Leigh questioned whether such protections would survive a challenge at the European level.
"The Government might think that any legislation it introduces is bomb proof, but the reality is the UK has a very poor record when tested in the European Courts," he said.
"Given the Government's poor track record of winning in Europe, it would be the height of arrogance to think any legislation will not end up before the European Court, where is stands a good chance of being ruled against."