Although the term king of Canada was used as early as the beginning of the reign of George VI, it was not until 1953 that the monarch's title was made official, with Elizabeth II being the first monarch to be separately proclaimed as Queen of Canada, as per the Royal Style and Titles Act.
The term king of Canada was used from the start of George VI's reign but didn't become officially recognized until 1953. At that time, Elizabeth II was specifically declared Queen of Canada in accordance with the Royal Style and Titles Act.