A letter criticising the Scottish government for supporting plans to legalise gay marriage will be read out in all of the country's Catholic parishes on Sunday.

The Roman Catholic church has declared 26 August as "national marriage Sunday" and is calling on politicians to "sustain rather than subvert marriage".

The letter, which will be read in all of Scotland's 500 Catholic parishes, will urge followers to continue to act against efforts to "redefine" marriage.

The Scottish government has said it is right to introduce same-sex marriages, but has stressed no clergy would be forced to carry them out.

Cardinal Keith O'Brien, the leader of the Church in Scotland, last week broke off discussions on the issue with the first minister, Alex Salmond.

In a strongly worded message, the letter will highlight the church's "deep disappointment that the Scottish government has decided to redefine marriage and legislate for same sex marriage".

It will also announce the launch of a national commission for marriage and the family to co-ordinate a campaign against gay marriage.

O'Brien, who has described gay marriage as a "grotesque subversion of a universally accepted human right", said: "The church's teaching on marriage is unequivocal, it is uniquely, the union of a man and a woman and it is wrong that governments, politicians or parliaments should seek to alter or destroy that reality."

He added: "With this letter we will announce the creation of a national commission for marriage and the family, a body which will be charged with promoting the true nature of marriage, it will develop an online presence and produce materials and organise events which will help Catholic families to support and sustain marriage.

"While we pray that our elected leaders will sustain rather than subvert marriage, we promise to continue to do everything we can to convince them that redefining marriage would be wrong for society."

The Scottish government has pledged to bring forward a bill on the issue later this year, and has indicated the earliest ceremonies could take place by the start of 2015.

The Scottish Government should not give into the "anti-gay agenda" being pursued by the Catholic Church, a gay-rights charity said.

The Equality Network, which is campaigning in support of same-sex marriage in Scotland, said politicians should stand firm over the plans.

Tom French, the charity's policy co-ordinator, said: "It is increasingly clear that the Church has an anti-gay agenda that it wants to impose on the rest of society.

"We urge the Scottish Government to stand firm on plans to introduce equal marriage and not give in to demands that would discriminate against LGBT people."

"In particular, the Equality Network would be deeply concerned at any attempt to promote an anti-gay agenda in schools. School should be a welcoming environment for all young people, regardless of their sexual orientation or their family situation."