Aston Villa captain Stiliyan Petrov has been diagnosed with acute leukaemia, the Premier League club have announced.
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.
"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."
Petrov - affectionately known as 'Stan' - was signed for Celtic by John Barnes and spent seven years at the Glasgow giants, during which time he won four Scottish Premier League titles.
He was handed the Villa captaincy by former manager Martin O'Neill, under whom he had previously excelled at Celtic.
The Bulgaria skipper took time to win over the Villa fans upon his arrival in 2006, before establishing himself as a key figure under O'Neill and his successors Gerard Houllier and current boss McLeish.
In 2008-09 Petrov was named players' player and supporters' player of the season at Villa Park and he remains a firm favourite in his native Bulgaria, for whom he has won over 100 senior caps since making his debut 14 years ago.
Petrov's team-mate Darren Bent immediately sent a message of support to the club's skipper on Twitter.
The striker wrote: "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."