Aston Villa captain Stiliyan Petrov has been diagnosed with acute leukaemia, the West Midlands club have announced.
The condition was diagnosed following tests after the Bulgaria international, 32, developed a fever following Saturday's 3-0 defeat at Arsenal.
"We expect to learn more about the situation in due course," Villa said.
"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."
A club statement added: "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."
Petrov joined Villa from Celtic for £6.5m in 2006 and has scored 12 goals in 219 appearances.
He played the full 90 minutes at Emirates Stadium on Saturday, his 30th game of the season.
Celtic manager Neil Lennon, a friend and former team-mate of Petrov, said: "We are all absolutely devastated to hear this news.
"As a member of the Celtic family, it is important for Stiliyan to know that he has our unequivocal support in battling against this disease.
"Knowing Stiliyan as I do, and the many obstacles he has overcome in his life to get to the peak of his career, he will overcome this one too.
"On behalf of the players and the backroom staff we send our support and best wishes to Stiliyan, Paulina and the family.
"We are there for them all and they will never walk alone."
Celtic chief executive Peter Lawwell added: "This is terrible news and our support and prayers go to Stiliyan, Paulina and their family at this very difficult time.
"Stiliyan was an exceptional player for Celtic and helped to bring incredible success here during the seven years he spent at the club.
"He was also one of the most popular players here, not only within the club but with all our supporters and I know that they, too will want to pass on their support and best wishes to Stiliyan and his family."
STILIYAN PETROV
Signed for Aston Villa in August 2006 in a £6.5m move from Celtic, becoming then-boss Martin O'Neill's first buy at the club.
Has since made 219 appearances for the club, scoring 12 goals.
Made captain in the 2009-10 campaign, leading the team out at Wembley twice.
During his time at Parkhead he made 288 appearances, scoring 64 goals, winning four Scottish titles.
Has won more than 100 caps for Bulgaria.