"He will get from Villa every ounce of love and support that we have, to help bring this to a positive conclusion," Villa said.
WHAT IS LEUKAEMIA?
Leukaemia is cancer of the white blood cells. Symptoms include pale skin, tiredness, breathlessness and susceptibility to infection. Acute leukaemia means the condition progresses aggressively and needs immediate treatment.
On average 2,500 adults are diagnosed with acute leukaemia in the UK each year, according to Leukaemia & Lymphoma Research
It is understood Petrov will begin treatment in London on Monday.
In the meantime the former Celtic midfielder is being seen by medical specialists.
"We expect to learn more about the situation in due course," a club statement continued.
"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."
"Such was his determination to learn English however that he took to working in a friend's burger van as a way to pick up the language and his fortunes turned as O'Neill deployed him in his more familiar central midfield role."
"We are relying on the very decent and experienced people in and around the club for support, guidance and leadership," he said.
Villa chief executive Paul Faulkner added: "I was completely stunned to hear this news about Stiliyan. After our game last weekend we initially thought that he was suffering from a virus and, like the true leader and consummate professional that he is, he had simply played through it."
Jonathan Fear of the
Vital Villa website
said: "There is absolute shock amongst the Villa fans, and the outpouring of love and admiration for Stiliyan increases by the minute.
"Villa fans will show that they care about him during tomorrow's game.
"When the chips are down, Villa is a family and Stiliyan is a large part of that."
Celtic manager Neil Lennon, a friend and former team-mate of Petrov,
he was "absolutely inconsolable".
"He will overcome this like all the other barriers he faced in his life.
"Stiliyan has the heart of a lion and as part of the Celtic family he deserves all our best wishes and support."
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.
Celtic chief executive Peter Lawwell added Petrov had been one of the club's most popular players: "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's a fit young man and everyone will give him the best chance and put him on road to recovery as soon as possible."
Professional Footballers' Association chief executive Gordon Taylor told BBC Sport: "It's just terrible news. I've just written to him offering our support and help. Our thoughts and prayers are with him. We've got our fingers crossed he'll pull through."
Aston Villa goalkeeper Shay Given:
"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."
Aston Villa striker Darren Bent:
"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."
Aston Villa defender Carlos Cuellar:
"I feel very sad with the news of our captain Petrov. Our support for him and his family. He is very strong and will win this battle. We are all with you my friend. #prayforstan."
Match of the Day presenter Gary Lineker:
"Terribly sad to hear that Stiliyan Petrov has been diagnosed with leukaemia. Thoughts are with him and his family. All cases of Leukaemia vary, but from personal experience with @GeorgeLineker we know it can be beaten #goodluckStiliyan."
Former Villa midfielder Ian Taylor:
"Terrible news re Stiliyan Petrov illness. Our best wishes go out to him and his family. Praying he can make a full recovery #getwellstiliyan."
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