A letter signed by more than 70 of Scotland’s largest evangelical churches will be handed over today to oppose the plans, reports the Scotsman.

The letter reads: “Redefining marriage is not an issue of equality or fairness, as argued by those calling for this change. Equality does not mean sameness but recognises diversity.

“In the Scottish Government’s consultation document it is abundantly clear that homosexual couples already have the rights of marriage available through civil partnerships.

“Our chief concern is that Scottish society will be the poorer if the definition of marriage is rewritten. Marriage is foundational and so much else rests upon it.”

It adds: “You cannot radically alter the definition of marriage without it having knock-on effects on so much else in our society.”

While a civil partnership gives gay couples the same legal rights as a married heterosexual couple, it is not recognised as a marriage in the traditional sense, as such.

The Scottish government’s consultation asking if gay marriage should be legalised addressed the possibility of allowing religious as well as civil ceremonies for same-sex partners.

The Rev David McCarthy, of St Silas Episcopal Church in Glasgow, told the Scotsman: “Traditional marriage gives kids the complementary parenting of a mother and father, which same-sex marriage does not provide.”

Scottish Muslims have also threatened a withdrawal of support for the Scottish National Party (SNP) should the definition of marriage be redefined to include gay couples.

Salah Beltagui, of the Muslim Council of Scotland, said: “Generally, support for the SNP [among Scotland’s Muslim Community] has been strong, but if this goes on, it might change things.

“If you go back a few years, Labour was getting the majority of the vote and after Iraq it eroded very strongly, so it takes one issue like that to change things.”

Beltagui added: “Our community as a whole are all of the opinion that the family should not be changed in definition. A family is a man and a woman and children.”

Conversely, the Equal Marriage campaign group has submitted about 18,000 responses to the consultation. The homepage of its website urges users to ‘Lift the ban on same-sex marriage in Scotland’ and ‘Take action now’.

The gay equality charity Stonewall Scotland is also urging members of Scottish parliament to support same-sex marriage.

The BBC reported this week that Scotland’s deputy first minister, Nicola Sturgeon, said all views would be listened to, but added that ministers “tended towards the initial view” that same-sex marriage should be introduced.