US actress Kristen Stewart has said she was acutely aware of the grief of the Dukes of Cambridge and Sussex while playing the role of their mother Diana, Princess of Wales in new film Spencer.

The highly anticipated movie finds Diana at breaking point in her marriage to Charles but obliged to spend Christmas with the royal family at Sandringham.

Much of the film centres on Diana's relationship with the young princes and Stewart said she was anxious to be respectful of them.

She said : "They’re in the movie, they’re the most important part of the movie, they are so character-defining.

"It’s funny to say character-defining, she’s not a character, and I could never be her for real, we could never know her completely.

"Ironically she’s a very unknowable figure, which is her plight, it’s sad, she just wanted to be known and to be available and she was to a certain extent, but she’s in a uniquely singular position.

"It’s impossible to relate to.

"But her kids…This is a movie. We’re so inspired by her.

"We genuinely love her so much.

Kristen Stewart on the red carpet at the Venice Film Festival

"It’s the only reason this movie was made, to give the idea of her another platform and give her a voice, give her a mouthpiece that she wanted so bad and did ultimately have and create for herself.

She added: "I think her legacy is so present in the way that those two men function in the world.

"They’ve both made very different choices. That’s her.

"She made mistakes, and she made choices.

"And the royals are not supposed to do either of those things.

"And you can see they function so positively in the world. I look at the both of them, and maybe I’m biased and sort of entrenched in this thing, but I look at them and I see her and that is a beautiful thing."

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Stewart also said she was so "debilitatingly petrified" about playing Diana that she suffered a physical reaction before shooting started.

She said: "I had this really intense TMJ (temporomandibular joint disorder, a condition affecting the movement of the jaw), I couldn’t open my mouth," the actress recalls.

"And as we all know, when Diana speaks, it’s so open.

"So at this point, I’d done all my research, I had prepped, prepped, prepped, and I wasn’t scared and it was totally worth doing this and I was totally ready, yet I can’t open my mouth.

"So I think I was kind of trying to tell myself that I wasn’t scared, but was clearly debilitatingly petrified.

"But then I woke up on the day first day of shooting and suddenly was just able to move.

"But I wasn’t sleeping. I knew about the film for almost actually two years before we started filming and I will tell you that there wasn’t one morning or one night where I went to sleep or woke up not going 'At some point I’m going to have to actually do this.’

"It’s all well and good to think about, it’s fun, and it’s cool, but it was one of those morning loomers for a really long time."

Spencer is out now in Irish cinemas