The Emmy-award winning star also co-created historical drama series The House Of Eliott.
Her friend, director Michael Lindsay-Hogg, confirmed her death in a statement on Sunday issued through her agent.
"Jean died peacefully in bed looked after by one of her very loving carers," he said.
"You could say we were very close for 60 years. She was as wise and funny as anyone I ever met, as well as being very pretty and kind, and talented as both an actress and writer.
"An instinctively empathetic person who was loved by everyone who met her. We spoke on the phone almost every day for the past 40 years."
She was best known for playing the role of Rose the pragmatic head house parlourmaid in the British drama television series Upstairs, Downstairs, which she co-created with Eileen Atkins.
For her portrayal she won an Emmy at the 1976 awards ceremony in the category of outstanding lead actress in a limited series.
Born Jean Lyndsey Torren Marsh on July 1, 1934 in Stoke Newington, north London, her mother worked in a bar and as a theatre dresser while her father was a handyman and printer's assistant.
She became interested in performing after taking dance and mime classes as therapy for an illness and began acting on stage, with a stint at Huddersfield Rep in the 1950s.
It was not long before she transferred to London and at the age of 12 the actress made her West End debut in The Land Of The Christmas Stockings at The Duke of York's Theatre.
Her earliest screen appearances came in such TV classics as The Twilight Zone and Danger Man.
She also appeared in Doctor Who adventures, most notably as William Hartnell's short-lived companion Sara Kingdom.
Her most notable films are fantasy adventure film Willow (1988), thriller Frenzy (1972), and war movie The Eagle Has Landed (1976).
A minor stroke forced her to take a break in 2011 but she returned to work afterwards.
She was married to Doctor Who actor Jon Pertwee for five years before their divorce in 1960 and she also had relationships with actors Kenneth Haigh, Albert Finney and Lindsay-Hogg.
She starred in a number of other TV series including Sense And Sensibility, Hawaii Five-O and Murder, She Wrote.