Many kings and princes have taken mistresses, but it is only really in modern times that they have psuhed for marriage.
Edward VIII and Mrs Simpson.
Prime Minister Baldwin sought legal advice and was told that the King's wife is always de facto Queen whatever title she may use - advice still relevant today.
In 1936 Edward VIII proposed to the British government that he be allowed to enter into a morganatic union (morganatic means that it prevents the passage of the husband's titles and privileges to the wife and any children born of the marriage.) with the American-born divorcee Wallis Simpson.
The idea was rejected by the British Conservative government, led by Stanley Baldwin, and by a clear majority of what were then called the Dominions (including Canada, Australia and New Zealand). After the rejection of his proposal, Edward VIII abdicated and was succeeded by his younger brother, who became George VI (Queen Elizabeth's father)
Princess Margaret and Group Captain Peter Townsend
There was a putative marriage of Princess Margaret and Group Captain Peter Townsend in the 1950s. Townsend was divorced. The government was advised that members of the royal family couldn't marry in a registry office (other documents suggest they later offered a compromise, with Margaret keeping her title but losing the right to succession).
Under the Royal Marriages Act of 1772, the Princess required her elder sister's permission to marry before the age of 25 - she would have been 25 in August that year. After the age of 25, she needed Parliament's approval, and Parliament had already made clear it would not consent to her marriage.
News of the proposed marriage was leaked in the press. Shortly after Group Captain Townsend was sent to Brussels for two years as air attaché at the British Embassy.
She was left with only two options: renounce all her rights and privileges and become Mrs Peter Townsend or give up all ideas of the marriage. She gave up ideas of marrying Townsend. Later she married another (Earl of Snowdon) and was divorced from him.
The present situation of Camilla as Prince Charles wife
The Duchess of Cornwall legally holds the title and rank of Princess of Wales as she is the wife of the Prince of Wales, but she does not, by choice, style herself as such.
Once Prince Charles ascended the throne as King, technically as the wife of a king, Camilla would be Queen. However, according to a Clarence House statement, it is "intended" that Mrs Parker Bowles should use the title the "Princess Consort" when Charles becomes king.
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