The actor died of complications from a brain tumour, according to a report in The Guardian, who noted that she died “suddenly” on Wednesday.
Nicola will be best remembered for her performance as Elizabeth Bellamy in the Emmy and Bafta-winning period drama.
She played the rebellious daughter of Richard and Lady Marjorie in two series of the ITV show, eventually bowing out in 1975, when her character moved to America.
Nicola’s other TV credits include The Sweeney and a leading performance in a BBC adaptation of Anna Karenina in 1977, as well as the Australian mini-series The Timeless Land and sitcom Ain’t Misbehavin’.
On the big screen, she appeared in films including Frankenstein: The True Story, There’s A Girl In My Soup and An Awfully Big Adventure, which also starred Alan Rickman and Hugh Grant.
Her final on-screen credit was in the TV mini-series Up Rising in 2000.
In addition to her on-screen performances, Nicola also had a successful career in the theatre, making her London stage debut in A Boston Story in 1968.
She would go on to appear in productions of A Voyage Round My Father and What The Butler Saw.
Nicola also won praise for her performances in Harold Pinter plays Party Time and Old Times, and was directed by the playwright in an adaptation of The Trojan War Will Not Take Place.
In 1997, Nicola released a book, Diamonds Behind My Eyes, in which she detailed her own mental health problems.
She is survived by her daughter Eve, whose father is Nicola’s ex-husband Graham Swannell, and her sister, Angela.