The Church of Scotland's General Assembly, the annual conference of 850 church leaders, will vote on the issue of homosexual clergy tomorrow.
The Free Kirk, which is opposed to the change, warned Christianity was “under siege” from the rise of secularism and criticised the Church of Scotland for failing to speak with a unified voice.
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Accompanied by the Duke of Edinburgh, the Queen will attend the opening ceremony of the fifth session in the Holyrood chamber.
Ms Leadsom has received the backing of Ian Duncan Smith, the former work and pensions secretary.
The former shadow work and pensions secretary is considered a more viable candidate than Angela Eagle, a Labour source has told ITV News.