Aston Villa have announced that midfielder Stiliyan Petrov is suffering from acute leukaemia.
Villa captain Petrov, 32, has made 30 appearances for the club this season in all competitions and most recently featured in Saturday's 3-0 defeat to Arsenal, but reported that he was feeling unwell in the aftermath of the game.
A club statement read: "The Villa board received news today that our long-standing captain Stiliyan Petrov has been diagnosed with acute leukaemia.
"Stiliyan developed a fever following the Arsenal game last Saturday and subsequently underwent tests. Haematology experts confirmed the diagnosis today.
"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.
"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."
The announcement brought an immediate outpouring of support for Petrov, with Celtic manager and former Hoops team-mate Neil Lennon among those taking to Twitter.
"Absolutely inconsolable here regarding Stilian," Lennon Tweeted. "He will overcome this like all the other barriers he faced in his life.
"Stilian has the heart of a lion and as part of the Celtic family he deserves all our best wishes and support."
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."
Jermaine Jenas, a former team-mate of Petrov's at Villa, said: "Just saw the news about Stan Petrov one of the nicest guys in football! Thoughts with him and his lovely family."
PFA chairman Gordon Taylor also promised all the help the players' union can provide to Petrov and has written to the Villa skipper to offer his support.
Taylor said: "It's a terrible shock, especially so soon after what has happened to Fabrice [Muamba]. All our thought and hopes with him.
"Stiliyan is a great character, a good leader, and we will give him all the help and support we can.
"Whenever I have met him I have been most impressed by him, he is a great gentleman, we are proud to have him as a PFA member and I just hope that, like Fabrice, he can come through it."