Princess Royal is a style customarily (but not automatically) awarded by British monarchs to their eldest daughters.

Although purely honorary, it is the highest honour that may be given to a female member of the royal family.

Princess Mary (later Queen Mary II) (1662–1694), the eldest daughter of King James II, and Princess Sophia Dorothea (1687–1757), the only daughter of King George I, were eligible for this honour but did not receive it.

Thus, Princess Louise was granted the style of Princess Royal by her father King Edward VII in 1905; she retained it until her death in 1931, over twenty years into the reign of her brother King George V. Only upon Louise's death did the title become available for George's own daughter, Princess Mary, who was granted the title in 1932, retaining it until her death in 1965.

Because Mary outlived not only her father but also her brother King George VI, the title was never available during George VI's reign to be granted to his elder daughter Princess Elizabeth (later Queen Elizabeth II), though she would otherwise have been eligible to hold it.