Professor Kate Williams told Channel 5's royal documentary 'Queen Mary: How she saved the Royals' that Queen Mary was greatly shocked by the death of her son. The royal expert added that Queen Mary was "broken" by the death of George VI despite her granddaughter Queen Elizabeth II replacing him on the throne.
Professor Williams said: "The death of George VI in 1952 was a great shock to Queen Mary.
"No one expected him to die so young.
"I think she was quite broken by the death of her son and also the end of his reign into which he had put so much effort.
"I do think that the death of George VI had a great effect on Queen Mary and really you started to see a rapid decline in Queen Mary.
"Even though it was her beloved granddaughter on the throne and that she had been devoted to Elizabeth since she was a little girl."
During the same documentary, Professor Kate Williams and Editor of Majesty magazine Ingrid Seward discussed Queen Mary's influence on a young Princess Elizabeth.
Professor William revealed that Queen Mary took Queen Elizabeth II and Princess Margaret to galleries because she wanted them to become more educated on the history and geography of the United Kingdom.
Ms Seward stated that Queen Mary taught Elizabeth and Margaret the "royal wave" and the posture expected of a Princess.
Professor Kate Williams said: "Mary felt very strongly that Elizabeth should be having more education in history and geography, learning about her country.
"She took it upon herself to take Elizabeth and Margaret out to galleries to really try and show them around.
"This is because Elizabeth's parents were a little bit distrustful of education."
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Ms Seward added: "The first thing she taught young Princess Elizabeth was to sit up straight and to stand straight.
"She made sure the pockets of the Princess' coats were sown up so she couldn't put her hands in her pockets.
"She also taught them the royal wave with the handkerchief.
"Elizabeth said to Margaret that 'handkerchiefs are for waving not blowing your nose on'."