Peggy Mitchell actress Barbara, who is 80, was given the news in April 2014 but her husband Scott Mitchell only revealed the sad news now.
Mr Mitchell said a small circle of friends who had begun to notice his wife's occasional confusion had been told shortly after the diagnosis.
Speaking about Barbara's diagnosis for the first time, Scott, 55, revealed to The Sun that his wife has been taking medication to manage her symptoms - but tragically her memory loss has worsened.
Peggy went through a string of on-screen husbands and fights between her notorious sons - Phil Mitchell (played by Steve McFadden) and Grant Mitchell (Ross Kemp). I'm lucky that my granny still knows who I am, for now. She knows her history and she knows what she has done'.
Barbara has been in the public eye for more than six decades. But this is the worst possible thing you can do to someone with Alzheimer's, as I learned from reading psychologist Oliver James' brilliant book, Contented Dementia.
She underwent rigourous testing which included a brain scan and a lumbar puncture to confirm the upsetting news for the couple.
In 2009 she received a Lifetime Achievement Award at The British Soap Awards. I hope by talking openly it will make it easier for others to talk about this terrible disease.
He added: "I love both Barbara and Scott very much".
And he wasn't the only one referring to Barbara in the past tense, as Susanna had to correct herself when she said how close Richard was to the star.
"He rang me last week and said, "I can't protect her anymore".
Good Morning Britain host Piers Morgan was among the others sending messages to Dame Barbara.
Mr Parry said there are more than 850,000 people in the United Kingdom living with Alzheimer's and other forms of dementia. Alzheimer's is a terrible thing.
After shedding some tears, her first words were: "I'm so sorry", Mr Mitchell said. We're all rooting for you, Babs!'
One tweeted: 'Why are GMB talking about Barbara Windsor in the past like she's dead?
"Ironically, I think it's some of the best work she ever did".
Tim Parry, Alzheimer's Research UK director, has praised him for the decision to go public with the devastating news, something he said they have been left "saddened" by.