Once upon a time in 1772, the Royal Marriages act of 1772 included a little thing called ‘The Instrument of Consent.’
The act requires that all descendants of George II receive sovereign consent before they wed, otherwise the marriage would be invalid.
The Royal Marriages Act was ordered by King George III, who was King George II's grandson.
The law came after King George III's younger brother, the Duke of Cumberland, secretly married Lady Anne Horton, deemed to be the disreputable widow of a commoner.
In 2018, the Queen needs to offer her consent to the first six people in line to the throne. Because Prince Harry falls under the first six people in line to the throne, the Queen needs to offer her approval of the union, and thankfully she did. The Queen signed The Instrument of Consent in March, hence the wedding that will be happening in 3 days time.
Here is a copy of the letter of consent:
