Dame Harriet Walter has described Downton Abbey co-star Dame Maggie Smith as “completely unique and completely irreplaceable” following her death aged 89.

The British stars shared the small screen in the ITV drama, with Dame Harriet starring as Lady Shackleton, a friend of Dame Maggie’s character Violet Crawley, Dowager Countess of Grantham.

“Maggie is absolutely unreplaceable. She was a complete one off,” Dame Harriet told BBC Radio 4’s PM programme.

“She was adorable to me. She was always really nice to me, and she rang me up on occasions and said why didn’t I ring more often and I felt I really should have had the courage to be her friend.

“But I was a little bit nervous of her, I suppose, and I tried to think what that nervousness was, and it was that one wasn’t going to be entertaining enough, gossipy enough, to hold her attention.

“She wasn’t actually someone who wanted to be the centre (of attention), any more than any actor. Of course you want attention when you’re on, but behind the scenes, she was a listener.

“She was a games player, she joined in, she liked company, but she was incredibly private…we have to admit that only her close family and her lovers, who are now gone, really knew her, and we just sort of brushed against her if we were lucky.”

Dame Harriet listed a variety of roles from Dame Maggie’s impressive repertoire that made an impression on her.

“She has had me in stitches in Oscar Wilde. She has had me terrified when she was playing Hedda Gabler,” she said.

“She was wonderful on screen in things that perhaps people haven’t seen like there’s a film called From Time To Time, which was a Julian Fellowes, he directed it and adapted it from a children’s novel, and she was just terribly sensitive and lovely in A Room With A View.

“And then, of course, coming up to Downton Abbey, where she nicked every scene.

“To be fair, she got some of the best lines, but she earned those lines, her whole career inspired people to write fantastic things for her to say.”

Dame Harriet said the pair also starred in a radio play together but they “couldn’t get through it, we were laughing so much”.

She added: “As far as her talent and her work and her ability is concerned, there is nobody like her.

“Nobody can follow in her steps.

“She was completely unique and completely irreplaceable, and that’s why we’re all very sad.”