The singer was admitted to an unspecified UK hospital earlier, although there are currently no details on his condition.
The rest of Korn will still appear on Download's Main Stage at 6.30pm today. The band will play a shortened 30 minute set, with a variety of guests helping out on vocals.
Metallica, who are headlining straight after Korn, famously endured a similar card of fate at this same festival in 2004, when drummer Lars Ulrich was unable to play due to illness at the eleventh hour. Slipknot and Slayer were among bands that donated their stickmen to fill Ulrich's stool and the resulting show went down as a historical event in Donington's rich rock tapestry.
Furthermore, speculation is rife that Metallica frontman James Hetfield may be amongst the Korn Karaoke contenders, given that Jonathan Davies once stood in for him at a Metallica show, when he was unable to perform due to injury.