Leukaemia is a cancer of the white blood cells, which we need to fight off infections.
The 32-year-old has not trained since featuring against Arsenal on Saturday.
“The Villa board received news today that our long-standing captain Stiliyan Petrov has been diagnosed with acute leukaemia,” the club said in a statement.
“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.”