Aston Villa captain Stiliyan Petrov has been diagnosed with acute leukaemia, the club said in a statement on Friday.
The 32-year-old Bulgaria international has not trained since playing against Arsenal on Saturday.
The condition was diagnosed following tests after the player developed a fever following Saturday's Premier League match.
"The Villa board received news today that our long-standing captain Stiliyan Petrov has been diagnosed with acute leukaemia," the club said.
"Haematology experts confirmed the diagnosis today. We expect to learn more about Stiliyan's situation in due course. During this time we ask that Stiliyan's privacy is placed ahead of all inquiries.
"Stiliyan is cherished by many and he will get from Villa every ounce of love and support that we have to help bring this to a positive conclusion."
Photograph: Giampiero Sposito / Reuters