The film The Queen, which was written by The Crown creator Peter Morgan, was about the shocking death of Princess Diana and the events that followed.
In an interview with Radio Times, Mirren shared that she felt the need to write to the Queen after realising just how intense that period of time was for Her Majesty.
"I realised we were investigating a profoundly painful part of her life, so I wrote to her," she said. "How do you write to your queen? Was it Madam, or Your Highness, or Your Majesty?"
"I said: 'We are doing this film. We are investigating a very difficult time in your life. I hope it's not too awful for you'. I can't remember how I put it. I just said that in my research I found myself with a growing respect for her, and I just wanted to say that."
The actress would go on to win a BAFTA and an Oscar for her portrayal of the Queen.
While she never received a formal response from the Queen, Mirren did receive a letter from the Queen's secretary.
After reading the response, she said, "I was very relieved subsequently that I had written the letter."
Mirren believes the Queen has watched biopic
Mirren has previously revealed that she believes the Queen has watched her portrayal.
"At the time, it had never been done before, playing the queen. It was quite nerve-racking because I didn't know – no one knew – how the public would receive it, let alone the establishment in Britain," she told The Hollywood Reporter.
"But I got the sense that it had been seen and that it had been appreciated. I've never heard directly, and I never will," she added.
The star has previously shared on Good Morning Britain that she knows Prince William and Prince Harry have seen the film and were "very cool" about it.
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