Deputy First Minister Nicola Sturgeon has been warned she risks losing her Holyrood seat if she pushes ahead with plans to legalise gay marriage.
Adil Bhatti, a former member of the SNP's national council, claimed both Catholics and Muslims may not vote for Ms Sturgeon.
Mr Bhatti, who quit the party last year in protest after it voted to reverse its long-standing opposition to Nato, claimed the Deputy First Minister would "lose more than she would gain" in her Glasgow Southside seat.
"The gay community don't have that many votes compared to those that you would lose from the religious groups like the Muslims and the Catholics," he said
"There is an option the religious groups might not vote at all. I don't think the Muslim and Catholic population will vote for her. I can't say there will be a complete blackout but the majority will vote against her."
Last year Ms Sturgeon announced the Scottish Government planned to legalise same-sex marriage, declaring it the "right thing to do".
Mr Bhatti, a former convener of the group Scottish Asians for Independence, said: "When people think about same sex marriage they think of the SNP mainly - not any other party."
Humza Yousaf, an SNP MSP for Glasgow, said the leaders of all Scotland's political parties backed same-sex marriage.
He added: "Most imams I have spoken to understand that religious freedom cuts both ways - while legal safeguards for Mosques and those who do not want to solemnise same sex marriage are vital, there is an understanding that what others wish to do in their faith institutions is entirely their own business."