At a Privy Council meeting on Wednesday, the monarch issued her approval, in a declaration, to the nuptials of her "most dearly beloved grandson Prince Henry".
King George III, George II's grandson, ordered the now repealed 1772 act after his younger brother the Duke of Cumberland secretly married Lady Anne Horton, deemed to be a highly disreputable widow of a commoner. It means the monarch must agree to the marriages of the first six people in the line of succession.
Meghan Markle and Prince Harry were the targets of an alleged anthrax letter sent to their Kensington Palace home 10 days ago.
Markle was baptised by the leader of the Church of England earlier this month ahead of her marriage to Harry out of respect for the Queen's role as head of the denomination.
He later told BBC News: "The fact that I fell in love with Meghan so incredibly quickly was confirmation to me that all the stars were aligned".
Meghan Markle and Prince Harry have something in common that holds them together. He added, "Even though Meghan will have known the enemy was not real, I guarantee that she will have been petrified". Speaking to The Express previous year, Holly explained: "Everybody loves a royal wedding, of course they do, and Meghan Markle is just incredible".
The 36-year-old has since taken part in her first official public engagement with the Queen, joining her and other leading members of the royal family for the Commonwealth Day service at London's Westminster Abbey on Monday (12Mar18).
The lunch comes as it's said that the Queen is purposely setting aside time to spend time with her future granddaughter-in-law - presumably as a way to make Meghan feel welcome before joining the family officially.
Despite the recent security scare, the couple will adhere to the plans they have made and make a carriage procession around Windsor Castle.