AceShowbiz - "American Idol" featured a very special guest appearance in its new episode. In the Sunday, May 7 episode of the long-running singing competition show, newly crowned King Charles III and Queen Camilla greeted audiences from across the pond.

The outing saw judges Katy Perry and Lionel Richie broadcast live at Windsor Castle just hours after they performed at the king's coronation concert. Of the festivities, Lionel said, "What a party. What a party. It was unbelievable." Katy, who looked stunning in a strapless green gown, added, "It was incredible and we're here for the coronation concert at Windsor castle."

"Now we're trying to figure out what we can do to bring something different to the show, so I would like to…," Lionel continued before King Charles and Queen Camilla joined him in front of the camera. The King later jokingly asked Lionel how long they were planning to use the room for.

He then gushed over Lionel and Katy. "Thank you so much, you're brilliant performers," the 74-year-old monarch said, before inviting the two stars to the concert's after-party.

Meanwhile, Ryan Seacrest detailed the unexpected cameo from the royals in an interview with Entertainment Tonight. "I have to tell you, I was driving in and my phone rings and it's the executive producer and she says, 'Well, we've got some news,' " the host explained. "I thought maybe something happened [with a contestant]. She said, 'Well, the king and the queen are going to make an appearance!' "

The former co-host of "Live with Kelly and Ryan" added, "I said, 'This is so great. How?' 'Well I don't know. They're just gonna figure it out when Lionel and Katy are in some room in Windsor Castle.' " He then joked, "So apparently, Lionel and Katy are staying overnight? Like, I don't know what's going on. They've got separate wing."