The 'Lolita' star's son announced the sad news that his parent passed away due to pneumonia complications not relating to COVID-19, after a lengthy health battle.
The 'Lost Boys' actor hailed his mom ''an extraordinary woman who led an extraordinary life''.
He wrote on Twitter and Instagram: ''Early this morning my mother, Shirley Douglas, passed away due to complications surrounding pneumonia (not related to COVID19).
''My mother was an extraordinary woman who led an extraordinary life.
''Sadly she had been battling for her health for quite some time and we, as a family, knew this day was coming.''
Although Shirley's death was not related to the coronavirus, Kiefer had a message of support to those who have lost relatives to the respiratory disease.
He added: ''To any families who have lost someone unexpectedly to the Coronavirus, my heart breaks for you, please stay safe.''
As well as featuring in Stanley Kubrick's adaptation of the novel 'Lolita' in 1962, and 'Dead Ringers' in 1988, Shirley was known for her activism against the War on Vietnam, and the battle for public healthcare.
In 2012, she was honoured with a Queen Elizabeth II Diamond Jubilee Medal.
Whilst the Canadian-born star received an Officer of the Order of Canada in 2003.
Shirley is survived by Kiefer, 53, and his twin sister Rachel from her marriage to actor Donald Sutherland - who she was married to between 1966 and 1970 - and Thomas Emil Sicks from her first marriage to Timothy Emil Sicks.
Kiefer's passion for acting came from his mother
He recently recalled: ''I was raised by my Mum, and my Mum is a very successful theatre actor in Canada.
''I have a twin sister and she and I would finish school, and we would go to the theatre, do our homework, wait for my mum's performance to finish, and then we'd go home. That was our day.
''I spent so much time as a child in the theatre. I could tell that people who worked in the theatre were different.
''When I would go visit other friends, at their house, their parents and their friends weren't the same. They weren't just colourful, they weren't just funny.
''I knew I wanted to kind of live in that world, I didn't know what I wanted to do.''
And Kiefer's mind was blown when he saw his mom on stage for the first time.
He added: ''My mother did a production of 'Virginia Woolf', and it was the first time I had seen either one of my parents where I didn't recognise them anymore and it blew my mind. It was like magic, and that's when I first got excited about what I thought was interesting about acting.''
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