The family of former Celtic manager and player Billy McNeill have confirmed he has dementia.
Mr McNeill, who has been described as the club's "greatest ever captain", led the team which won the European Cup in 1967.
His family told a Sunday newspaper he was diagnosed with the illness seven years ago and is now unable to speak.
Celtic chief executive Peter Lawwell said Mr McNeill was facing his situation with "true bravery".
The former Hoops captain enjoyed a glittering career at the Parkhead club, where he became the first Briton to lift the European Cup after a 2-1 win over Inter Milan in Lisbon in 1967.
McNeill, 76, also led Celtic to nine successive league titles and won seven Scottish Cups and six League Cups, before having two spells as manager.
He also managed English clubs Manchester City and Aston Villa in the 1980s.
He is currently being cared for by his wife Liz, 73, at their home in Newton Mearns, Glasgow.
She has spoken to the Sunday Mail and the Scottish Sun on Sunday about her husband's battle with the degenerative brain disease.
She said he relies mostly on hand gestures to communicate his feelings.
She told the Sun on Sunday: "His concentration is not as good as it was and he now can't communicate very well. It's affected his speech over the last year or two.
"Sometimes, if something annoys him, he can still say a few words like 'don't do that'. But in general he finds it very difficult. It's not because he doesn't know how to speak. There's just a part of his brain that won't let him. I miss the conversation."
The McNeills decided to bring their experiences into the open as the 50th anniversary of the Lisbon Lions' historic victory in May 1967 approaches.
And they have backed a campaign by the Sunday Mail for more funding to further research links between dementia and heading a football.
Mrs McNeill said: "I think it's the right time for us to talk about this now. Heading the ball and the possibilities of concussive effects on the brain needs more discussion.
"We don't know if Billy's dementia is linked to his football. More research needs to be done."
Celtic chief executive Peter Lawwell said a proportion of the proceeds of a forthcoming Celtic FC Foundation charity match will go towards supporting people with dementia.
He said: "Billy is a man of true stature, someone who has given so much of his life to Celtic and at all times with such grace, humility and dignity.
"He is respected by all in football and someone who will always be loved dearly by all Celtic supporters."
'Very special man'
Mr Lawwell added: "We have always been in close contact with Billy and his family and have been well aware for some time of the challenges he has been facing, a situation he has met with true bravery, a quality synonymous with Billy McNeill.
"I know Liz has been an absolute tower of strength too for Billy. Her compassion and care for Billy has been crucial and I would like to pay tribute to her and the rest of the family.
"He is a very special man and everyone at Celtic will continue to give Billy and his family every support."
Footballer commentator Archie Macpherson told BBC Scotland Mr McNeill's diagnosis "underlines the importance of more research".
He said: "I saw him during the unveiling of his statue at Parkhead and he just about recognised me but wasn't able to converse too much. It was sad, enormously sad and I feel for Liz.
"I think the organisers and legislators of the game have got to look seriously into specific research in football - what effect heading the ball has on a footballer."
The commentator said research had been done into head injuries in rugby and American football but that football had been "a little bit too casual about this".
"I think it really has got to be specifically examined," he added.