Plans to legislate for same-sex marriage reach a milestone today when MSPs vote on the issue for the first time.

Holyrood is to debate the general principles of controversial Scottish Government proposals to allow gay couples to wed.

Ahead of that, Health Secretary Alex Neil urged politicians from all parties to back the change, arguing that "marriage is about love, not gender".

But some religious groups remain opposed to the move, with one church leader describing the legislation as a "future calamity in our nation".

The Reverend Paul Gibson, a minister with the Free Church, has sent an open letter to First Minister Alex Salmond urging him to abandon the proposals.

Rev Gibson, the minister of Knox Church in Perth, said: "Given that the overwhelming testimony of scripture declares that this proposed legislation is against the will of God, its passing will mark a dreadful day in the history of our nation."

The Marriage and Civil Partnership Bill aims to protect the rights of religious celebrants and groups opposed to allowing gay couples to wed.

Under the plans, religious bodies would have to opt in to perform same-sex marriage ceremonies. If a religious group does decide do this, protection will also be offered to individual celebrants who feel it would go against their personal faith to carry out gay weddings.

Mr Neil said: "At the heart of this issue there is one simple fact: a marriage is about love, not gender. I believe it's essential that all couples in Scotland know they have the same rights and privileges, and the same ability to get married.

"I hope that our national parliament takes this opportunity to send a clear signal that in a modern Scotland we are passionate promoters and defenders of equality, and I would urge MSPs to support the Bill."

Mr Neil stressed that he fully recognised and supported "the right of all members of the Scottish Parliament to vote on the bill as a matter of conscience".

But he added: "This Bill not only allows for same-sex couples to marry, it also puts in place protections on freedom of speech and freedom of religion.

"We are allowing those faiths who wish to perform same-sex marriages to do so while also protecting those faiths and celebrants who do not wish to take part."

Liberal Democrat leader Willie Rennie said today's vote on the Bill would be an "historic day for equality in Scotland and an important step towards a fairer society".

He said: "Equal marriage cannot come too soon for Scotland. It is wrong that people should be discriminated against simply because they happen to be in love with someone of the same sex.

"The message that this Bill sends is simple. Same-sex partners do not love one another any less than other couples. They deserve the same rights and protections as everyone else."

Equality campaigners also welcomed today's Holyrood debate on the issue, with Tom French, policy co-ordinator for the Equality Network, stating: "Today the Scottish Parliament has the historic opportunity to stand up for a fairer and more equal Scotland by voting for this milestone legislation.

"We urge MSPs to get behind this Bill, which will remove discrimination from the law and send out an important message about the kind of country we are."