The Princess of Wales, Kate Middleton, made a stunning appearance at Trooping the Colour on Saturday, June 15, despite her ongoing battle with cancer. According to royal expert Katie Nicholl, the mother-of-three's return to the spotlight was not without its challenges, as the public display took a significant toll on her health.

"The princess has this sort of swan-like quality about her -- she's sort of gliding on the surface -- but beneath the water, I think often she's paddling quite frantically," Nicholl told Entertainment Tonight. "I can only imagine that a huge amount of preparation and time and effort and energy will have gone into Saturday's appearance."

Nicholl noted that Kate was on her feet for an extended period during the event and even accepted a seat when offered, acknowledging that she is not as strong or fit as she was before beginning her cancer treatment. Despite the physical challenges, Kate "looked impeccable," and if one were unaware of her health struggles, "you would never have guessed," Nicholl added.

The decision to attend Trooping the Colour was Kate's alone, with no pressure from anyone else. "She really wanted to do this," Nicholl said. "She turned a corner in her treatment... When she turned that corner, I was told that Trooping was a goal for her. She was given the sign-off by her medical team. She spoke to her father-in-law about attending and to Prince William. They were fully behind her."

King Charles III has been a great source of support for Kate throughout her treatment, with Nicholl revealing that their already close relationship has grown even stronger. "Charles has been a great pillar of support to Catherine when she said that she'd really like to be there at Trooping," Nicholl said. "He was 100 percent behind it. The idea that she was going to upstage him didn't even cross his mind. He just wanted his daughter-in-law by his side."

Kate's appearance at the event was nothing "short of immaculate," according to Nicholl, who praised the Princess of Wales for her poise and the way she shepherded her children, Prince George, 10, Princess Charlotte, 9, and Prince Louis, 6, throughout the day.

"I think clearly a huge amount of effort went into her appearance behind the scenes, and it wasn't just about how she looked. It was how she held herself. It was the pride that she took in being there, the way she shepherded her children into the carriage and up onto that balcony appearance," Nicholl said.

While the successful appearance should "give us all a great sense of optimism" about Kate's health, Nicholl cautioned that it does not mean she will be returning to full-time work anytime soon. Kate has been open about the ups and downs of her cancer treatment, and Nicholl emphasized that the Princess of Wales will need time for recovery and privacy.

In the coming months, Kate is expected to attend a handful of engagements, provided she receives approval from her medical team and feels well enough. Nicholl suggested that Wimbledon, a tennis tournament that Kate takes great pride in attending as part of her role with the Lawn Tennis Association, might be a good event for her to attend.

"It wouldn't surprise me at all if we see her pop up at Wimbledon, because I think that is something that will bring her joy, and a huge amount of her recovery depends on her being happy," Nicholl said.