Celtic boss Neil Lennon has led the tributes to Aston Villa's Stiliyan Petrov after he announced his retirement from football.
Lennon spent six seasons playing alongside Petrov in the Celtic midfield, winning four Scottish Premier titles.
The Northern Irishman said: "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.
"It was a pleasure to play alongside such a great footballer, but, for me, I'm even luckier to call him my friend."
Celtic chief executive Peter Lawwell added: "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."
Petrov will lead out his team-mates and their families for their lap of appreciation following Saturday's home game with Chelsea and former Villa player Ian Taylor says it will be an emotional afternoon.
Taylor tweeted: Stiliyan has been a fantastic servant and role model for Aston Villa. The 19th minute is going to be amazing and emotional. Good luck Petrov."
A number of professional footballers have also tweeted their tributes to Petrov.
Former teammate Gary Cahill said: "He was fantastic. He captained me a few times when I was there.
"I am sure you will have heard already that he was not just a quality footballer but as a guy, a family man, he was top drawer.
"He has got a lot of friends in the game so it is sad to see that, but I wish him all the best. He was a really nice guy."
Fabrice Muamba said: "Petrov is a gentleman on and off the pitch. I speak from experience when I say things can only get better from here. God bless you and your family.
Manchester City captain Vincent Kompany said: "I wish Stiliyan Petrov all the best for this new chapter in his life. It's sad to see a great captain and a great man retiring in his prime."
Football Association chairman David Bernstein offered his own tribute to Petrov.
"On behalf of The FA, I would like to pay tribute to Stiliyan Petrov upon his retirement and reiterate our support as he continues to be treated for acute leukaemia," Bernstein told www.thefa.com.
"As a player for club and country, he was a tremendous ambassador for the game and I am sure the football family will continue to show their friendship and backing for Stiliyan.
"The news that he wants to launch a foundation to help others coping with cancer is testament to his character. I am sure the foundation will be a great success.
"Our thoughts at The FA remain with Stiliyan and his family."