The Scottish Parliament will be ignoring the public if it backs gay marriage in a vote at Holyrood, a leading opponent of the proposed move said.
Glasgow SNP MSP John Mason said a group of Nationalist politicians were still unhappy with plans to introduce same-sex ceremonies.
Holyrood is expected to back gay marriage in a vote on Wednesday, rubber-stamping the first stage of the process.
But Mr Mason is one of a handful of SNP MSPs likely to vote against the bill, which will go through another two stages before becoming law.
Health secretary Alex Neil said the SNP Cabinet was “united” in favour of the change.
However, energy minister Fergus Ewing is to stay away from the vote on the Marriage and Civil Partnership (Scotland) bill, due to family commitments.
Mr Mason said: “The parliament is not reflecting the views of the public. Parliament is out of line with what the public thinks. Clearly there’s a group within the SNP that’s not happy with this.”
First Minister Alex Salmond was also urged to abandon the plan by a church leader who warned in a last-ditch letter that the legislation was “a future calamity of our nation”.
Rev Paul Gibson, Free Church minister of Knox Church in Perth, said that Scots opposed gay marriage would be discriminated in the workplace or in applying for jobs, have their freedom of speech restricted and be denied access to public services.
In his open letter, Mr Gibson said: “Mr Salmond, it is simply not sufficient to say that these things will be a given.
“They must be enshrined in any legislation, since the testimony of history clearly shows that where these things are not explicitly safeguarded.”
Senior Labour figures refusing to vote for the bill include MSP Michael McMahon and deputy presiding officer Elaine Smith – both of whom are Catholics.
However, gay marriage is almost certain to be passed by MSPs with overwhelming cross-party support.
Mr Ewing, who is widely viewed as a socially conservative SNP member, refused to say whether he favoured same-sex marriage yesterday.
Humza Yousaf, Holyrood’s first Muslim minister, is among those backing the changes. Mr Yousaf, Scotland’s external affairs minister, said that he was “supportive” of the bill.
Labour MSP Mr McMahon said he would abstain to see if protections could be added to the bill to ensure churches were not forced to host gay weddings.
The Scottish Government has pledged that if the bill is passed, no church or religious celebrant will be compelled to conduct same-sex marriage weddings.
Mr Neil said the bill was backed by all SNP Cabinet members. He said: “The Cabinet is united on this and all the Cabinet will vote for it.”