Aston Villa captain Stiliyan Petrov has been diagnosed with acute leukaemia, the club have said.
The condition was diagnosed following tests after the Bulgaria international, 32, developed a fever following last Saturday's defeat to Arsenal.
A statement from the club said: "We expect to learn more about Stiliyan's situation in due course and we have moved quickly to support him and his family. During this time we ask that Stiliyan's privacy is placed ahead of all inquiries and trust that we will share information as we receive it."
The statement continued: "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."
Support poured in for Petrov from his Villa team-mates on Twitter.
Striker Darren Bent said: "Really sad news to hear about our captain Stan Petrov. hopefully he'll be back fighting fit soon. Thoughts are with him and his family. KTF."
Goalkeeper Shay Given tweeted: "In shock about Stiliyan Petrov acute leukaemia but we are all here for him and his family. Please God he makes a full recovery.#prayforstan."
Petrov's former Celtic team-mate and current Hoops manager Neil Lennon revealed his "absolute devastation" at the news.
Petrov spent seven years and won four Scottish Premier League titles at the Glasgow giants.
Lennon tweeted: "Absolutely inconsolable here regarding Stiliyan. He will overcome this like all the other barriers he faced in his life."