Britain's Prince Harry greets guests on the Mall as they attend the Patron's Lunch, an event to mark Queen Elizabeth's 90th birthday, in London, June 12, 2016. Reuters

Prince Harry won’t be able to get engaged to his girlfriend Meghan Markle until Queen Elizabeth II approves of their marriage first because of a United Kingdom law created in 1772. The law, called The Royal Marriages Act, gave the monarch the ability to veto the marriage of a relative who is within 6 family members away from the throne, People reported Thursday.

Prince Harry was fifth in line to the throne after Prince Charles, Prince William, Prince George and Princess Charlotte.

Prince Harry and Markle's engagement was “certainly one possible outcome," a source from Buckingham Palace told People. Harry has been dating Markle, the star of the USA show "Suits," since September 2016.

Though it has not been reported if Markle has ever met the Queen, another Buckingham Palace insider told Us Weekly in December that Her Majesty was “fully supportive” of her grandson’s relationship with the 33-year-old actress.

The two women could, however, meet sometime in the foreseeable future as they were both scheduled to be in London for the next few months. With season 6 of "Suits" being filmed in London, Markle has been spending a lot of time at Harry’s Nottingham Cottage home at Kensington Palace in the recent weeks. And the Queen, who returned to Buckingham Palace Tuesday after spending the winter at her estate in Norfolk, has high-profile meetings scheduled in the English capital in the coming months.

Princess Kate Middleton was not introduced to the Queen until the 2008 wedding of Her Majesty’s eldest grandchild, Peter Phillips, which was several years after Middleton and Prince William had begun dating.

The Royal Marriages Act was established after King George II became displeased with his younger brother, Ernest Augustus, in addition to losing the American colonies. George II created the law when his brother, The Duke of Cumberland, secretly married Lady Anne Horton, who the king saw as high disreputable and having “bewitching eyes,” according to BBC America.