"When Clive went into the dressing room he collapsed and was very seriously ill," said Leicester chief executive Tim Davies.
"He was taken away in an ambulance to hospital, he did offer a few words in the ambulance.
"Our thoughts and prayers are with Clive and his family - we hope he makes a speedy recovery. I would like to say a big thank you to Nottingham Forest for dealing with the matter so fast."
Clarke's collapse came on the same day that Spanish defender Antonio Puerta died after suffering a heart attack during Saturday's La Liga game between Sevilla and Getafe.
