Cardinal Keith O’Brien, the man that compared legalizing gay marriage with bringing back slavery, has declared war.

The leader of Scottish Roman Catholic Church has warned of an unprecedented backlash if Alex Salmond, Scotland’s first minister, presses ahead with plans to legalize gay marriage.

The Sunday Times reports O’Brien has revealed a strongly-worded letter to be read out on 26 August, on a day he calls ‘Support Marriage Sunday’, in each of the church’s 500 Scottish parishes.

It will urge Catholics to donate money to help fund a £100,000 (€126,089, $154,750) advertising campaign against the plans.

O’Brien said: ‘Marriage is under threat and politicians need to know the Catholic Church will bear any burden and meeting any cost in its defense.’

‘We will use this opportunity to remind Catholics of the importance of marriage as a union of a man and a woman and to urge them to be generous in contributing to a special collection which will be used to support initiatives in defense of marriage,’ he added.

Scotland’s ministers will announce the results of the wide spreading consultation this month, with a finalized bill likely to appear by 2013.

Unlike England and Wales’ legislation, it is planned Scottish churches will be able to bless gay unions but would be free to opt-out.

A Catholic Church spokesman said: ‘This is a straight-forward plan by the church to up the stakes in the war on gay marriage.’

O’Brien has tried to stop marriage equality before, describing same-sex marriage as a ‘grotesque subversion of a universally accepted human right’.

He has claimed gay marriage would be forced on Catholic churches and used discredited research to say same-sex relationships are unhealthy and inferior to straight ones, especially in bringing up children. He has also said schools will be forced to stock ‘homosexual fairy stories’ in their libraries.

In a recent Scottish Social Attitudes Survey, there was 61% support for the legislation and just 19% opposition.