The 37-year-old "So Hott" singer was hospitalized after feeling ill moments before he was to take the stage Friday afternoon at the three-day Download Festival, NME.com reported Saturday.
"We've had the curse of Download. Kid Rock is not very well, we've had to rush him to hospital in Nottingham. He apologizes to his fans," the crowd was told.
His representative later told reporters it was due to dehydration and stomach cramps, the entertainment news Web site reported.
People magazine reported British media said Rock, whose real name is Robert James Ritchie, had been up until the early morning at an event at London's Whisky Mist nightclub.
The singer had recently told the media he had repaired his relationship with his former wife, "Baywatch" star Pamela Anderson, whom he was once married to for four months, People reported.
"If I see her, I say hello, but I kind of don't want to be on that scene." Rock had said of the curvaceous star. "I touched the stove, it's (expletive) hot, I don't want to touch it any more."