According to multiple British media outlets that say they're in the know on this, Charles and co. have whiffed on some major players that could've potentially been booked for the May ceremony ... where KC will be officially crowned as the new sovereign in all his glory.

The Sun reports that 3 HUGE stars apparently told the Royals, of late, that they're unable to attend and/or perform ... including The Spice Girls, Harry Styles and Sir Elton John (who's a knight, no less). The reason for preemptively getting out of it ... conflicting schedules.

In Elton's case, he's on his farewell tour ATM ... and around early May (when the coronation goes down) he's actually scheduled to do a slate of shows in Germany before and after the ceremony. Per the Sun, going to and from in that short window isn't feasible.

As for Harry -- who's also touring, and never seems to really stop -- he technically has availability in early May to potentially squeeze in a BP gig ... but the Sun says it's going to be too hard for him to get bandmates together for that, and that they need "downtime."

And, when it comes to the Spice Girls ... apparently, it's too many schedules for too many ladies. That's about all the Sun has to say about that -- so, they ain't performing either.

Of course, there's also Ed Sheeran and Adele ... whom the Palace also reportedly approached, only to be shot down. DM reports both told the King they couldn't do it -- with Ed citing touring conflicts, and Adele apparently not offering (or having) a specific reason.

So, who's actually going to do the gig? At this point, the British media seems to be in agreement that Lionel Richie, Take That (minus Robbie Williams), Olly Murs and a handful of other not-as-famous music acts are in line to step up and carry the torch for HRH.