The newly appointed King joined his siblings, Princess Anne, Prince Edward and Prince Andrew as they mounted a vigil around the Queen’s coffin ahead of her state funeral on Monday.
All four siblings wore military uniforms as they made their way to the catafalque and each stood on all four sides for around 15 minutes.
Meanwhile, mourners filed into the Hall to make their way through to pay their respects.
King Charles looked solemn as he stood beside the coffin of his mother.
Body language expert Judi James said: “Charles led his siblings out with his head bowed and during the vigil, it was possible to see his performing several ‘coping’ rituals, like sucking in of the lips to increase his levels of determination, as well as closing his eyes and re-setting his jaw.
“As he paused facing the coffin, both his hands fiddled and worked away, even touching the sides of his coat in a small but telling self-comfort or anxiety ritual.
“In the moments before he adopted his vigil pose his hollow eye expression and frequent blinks were another testament to his tiredness and perhaps his closeness to tears.”
Ms James noted that the current monarch looked “exhausted” and had a “heavier gait” when compared to his sibling’s more stoic, military-style walks.
She said: “Anne looked around the hall with rounded, haunted-looking eyes as she came down the stairs but as she paused with the others before taking her post.
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“We could see she and her brother Edward banging their heels down on the floor as though standing to attention and, again, this military gesture of strength and precision could have helped them cope and control their expressions of grief.”
The body language expert said Prince Edward had the most “rigid poker face” of the Queen’s four children.
But there were telltale signs the Earl of Wessex found the vigil difficult, she said.
Ms James said: “He looked so much younger here in his uniform and so quietly and calmly determined to get everything right.
“He gave little away via his body language apart from a small sign of tension and the emotion cost of the vigil when a muscle was working away in his jaw”.
Commenting on Prince Andrew, who was given the green light to wear his full military uniform, said it allowed him to “wear a rather more emotional facial expression”.
He said: “With the signs of grief, like a hollow, reflective eye gaze and a down-turned mouth, more obvious than they had been when he adopted the rather splayed chest and lifted chin of dutiful bravado and resilience while in civilian dress.”
Queen Elizabeth II’s state funeral will be held at Westminster Abbey on Monday, September 19.