Gordon Wilson, a former leader of the SNP, is to speak out against same-sex marriage as the Scottish Government ploughs ahead with its controversial move.
Mr Wilson is to make the comments on Thursday at an event organised by campaign group Scotland for Marriage during the Scottish National Party conference.
The former leader will accuse the Government of ignoring its own consultation on the issue in which two thirds of respondents were opposed to same-sex marriage.
He will say: “What is the point of canvassing the views of the electorate and then cynically discarding them?”
And he is set to comment, “if you think we are sliding down a road to state fascism and intolerance, you may not be far wrong”.
Mr Wilson is to say: “Be warned, those MSPs of all parties on a narrow majority who exercise their ‘free’ vote for same sex marriage.
“They may find that a free vote has as much validity as a ‘free’ lunch. If they ignore public opinion, a P45 may await. Do not scorn the electorate.”
Mr Wilson, who led the party from 1979 to 1990, told BBC Scotland that political correctness was now occupying a position “to which it has no right”.
He added: “What you have to watch is when the state believes that it has a superior claim to tell its own citizens what to do and what to think.
“And that is what is beginning to happen here.”
Scotland for Marriage is running a petition on the issue which has been signed by over 30,000 people.
People living in Scotland can sign the petition, which is available here.