Miriam Margolyes has emotionally discussed watching her father become "more of a shell and a husk" as Alzheimer's took hold.

The 'Harry Potter' actress initially tried to ignore the feeling that something was "wrong" when her elderly dad Joseph first started showing symptoms of dementia and found it "utterly devastating" when she eventually saw only a few "glimpses" of the parent she'd grown up with as the condition progressed.

Speaking out about her late dad in support of Alzheimer's society, she said: “When my father developed his symptoms in his early 90s, I had a feeling that something was wrong, but I tried to push it away.

“It was only when neighbours phoned me and said Daddy was on their doorstep saying that they were in his house, that I realised he couldn’t be left on his own anymore and that Alzheimer’s – I didn’t know what it was then – but the illness that had taken hold of him had a vice like grip, and my father was no longer the man I knew. It was utterly devastating.

“Every now and again I had glimpses of the Daddy who had looked after me and had been my support and my rock. But very little; he became more and more of a shell and a husk.

“It wasn’t fair because he was a good, kind, loving man and he just seemed to disappear in front of my eyes. I couldn’t deal with it – I didn’t know what to do or who to ask.”

Miriam, 82, acknowledged dementia has a huge impact on relationships and she feels reassured that whatever happens in her own life, she has a supportive bond with her partner Heather Sutherland, who she has been with for over 50 years.

She added: “Unfortunately as you get older and when illness strikes, the whole basis of your relationship changes. Dependency changes, expectation changes, capacity changes.

“The person that you fell in love with and longed to be with when you began your love affair, then at the end of your lives, there is a stark realisation that things have changed utterly and forever.

“I have been with my partner for 54 years and I hope we will be until we are both corpses!

“I think it is just joyous to know someone that well, to trust someone that much and to know that you are loved. Life is sweeter shared.”

Miriam shared details of her father’s experience to highlight a series of adverts - narrated by Colin Firth - featuring couples affected by dementia who have vowed to stay together in ‘sickness and in health’.

Visit alzheimers.org.uk for more information or to donate.