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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Temperatures will reach around 28 Celsius (82F) at their highest.
A minute's silence will be observed at 11am on Thursday in remembrance of those who lost their lives in the Manchester Arena attack.
New research has found those who consumed two cups had a 35% reduced risk, and for those who drank five cups, the risk halved.