Celtic manager Neil Lennon has paid tribute to the "absolute courage and positivity" of Stiliyan Petrov after his former Parkhead team-mate announced his retirement from football.
The 33-year-old Aston Villa captain has declared himself ready to embrace new challenges in a bid to make a difference to the lives of people who, like himself, are diagnosed with leukaemia.
Lennon has remained close friends with Petrov since the Bulgarian departed for Villa in 2006 and saluted his strength in his battle against acute leukaemia. Petrov is now in remission after being diagnosed with the illness in March last year.
The former Celtic captain told his club's official website: "Obviously it's very sad that Stiliyan has announced his retirement from football.
"He was a fine footballer and enjoyed a magnificent career at Celtic and Aston Villa, as well as representing and captaining Bulgaria on many occasions.
"First and foremost, of course, is his health, and the news earlier this year that Stiliyan is in remission was a great boost for everyone.
"I'm sure that Stiliyan's characteristic determination, along with the fact he's surrounded by a loving family, will ensure he's a success in this next stage of his life.
"The words he has spoken today demonstrate his absolute courage and positivity and show exactly the kind of man he is.
"It was a pleasure to play alongside such a great footballer, but, for me, I'm even luckier to call him my friend."
Petrov was barely out of his teenage years when John Barnes signed him for Celtic in 1999 but he matured quickly to form an integral part of Martin O'Neill's successful team, winning four titles and four domestic cup winners' medals as well as appearing in the 2003 UEFA Cup final.
Celtic chief executive Peter Lawwell said: "The thoughts and best wishes of the whole Celtic Family are with Stiliyan, as they have been ever since we first heard the terrible news about his illness.
"I think the way he has coped with this and battled it, is in keeping with the type of character he is.
"Stiliyan was a great Celtic player in a great Celtic team who achieved wonderful success for the club, both in Scotland and in Europe, and from the time he arrived in Glasgow back in 1999 as a 20-year-old until he left seven years later, and beyond that, he has remained an extremely popular figure with the Celtic support.
"We all wish Stiliyan, Paulina and their family well as he retires from football and begins the next stage of his life."