Kate Middleton had a firm command for her youngest son, Prince Louis, during Saturday’s Trooping the Colour ceremony, according to a lip reader.

The Princess of Wales, at age 43, looked stunning while standing next to her husband, Prince William, and their three children at King Charles’ official birthday festivities.

Toward the end of the event, the royals headed up to the Buckingham Palace balcony for their annual smile-and-wave moment.

While Louis mostly behaved himself with the help of his siblings, George, 11, and Charlotte, 10, during the event, Princess Catherine did give a straightforward instruction to her 7-year-old son.

As the royal family assembled at the platform to observe the Royal Air Force’s flypast, the princess was noted, by lip reader Jeremy Freeman, to instruct Louis to stand at attention for the national anthem.

The mom of three said, “National anthem. Louis, stand,” Freeman told The Mirror.

Freeman exclusively told The Mirror that the young royal was looking restless as the festivities drew to a close, prompting Kate to say to her children: “National anthem. Louis, stand.”

Louis immediately straightened out his posture and turned his attention to the national anthem.

In addition to his parents and siblings, Louis was joined by the king, 76, and Queen Camilla, 77, on the balcony of Buckingham Palace for the end of the event.

Elsewhere on the balcony, the monarch and William appeared to be in awe of the stunning military parade as well as the Red Arrows aerial show.

According to Freeman, the future king turned to his father after the RAF flypast, saying, “Oh my word, what a day.” His father responded, “Great theater.”

During another sweet moment, the king had a candid chat with the Prince and Princess of Wales, as well as Prince George.

“What is that?” Charles asked his son, who was chatting with Princess Kate at the time.

As the king pointed to an aircraft flying over the balcony, William responded, “Er, is that Poseidon?” 

Prince George — who is currently learning to fly — chimed in, confirming, “It’s a Poseidon, yeah.”

This year’s event was a defining moment for King Charles as he showed up despite dealing with ongoing health issues.

His Majesty sent a message of unity by attending his celebration and standing in front of the British public while still undergoing cancer treatment after revealing his diagnosis in February 2024.

“It’s a show of strength,” a palace source told The Post. “The King is serving his country, and he’s not going to let a diagnosis get in the way of that.”

Charles’ former personal butler, Grant Harrold, also exclusively told The Post, “For the King, Trooping has always been important. It’s when all the royal family get together to show a united front.”