Nicola Pagett, a British stage and screen actress who dazzled millions of television viewers as Elizabeth Bellamy, a headstrong daughter of Edwardian aristocrats who grows up to become a militant suffragette in the acclaimed period drama “Upstairs, Downstairs,” died March 3 at a hospice center in Esher, a London suburb. She was 75.

The cause was brain cancer, said her daughter, Eve Swannell. Ms. Pagett had been diagnosed with the disease less than three weeks earlier.

Ms. Pagett performed in plays by Harold Pinter, Shakespeare and Molière — “great minds,” she once said, “that rub off into your everyday life” — and had already shared the stage with Vivien Leigh and Alec Guinness by the time she turned 30. But she was perhaps best known for playing strong-willed aristocrats in television shows that made her famous on both sides of the Atlantic.

As Elizabeth Bellamy, Ms. Pagett played a central role in early seasons of “Upstairs, Downstairs” (1971-75), which chronicled the decline of the British aristocracy by focusing on the Bellamy family and their servants. While Elizabeth’s father Richard Bellamy (David Langton) rules “upstairs” at 165 Eaton Place, a lavish London townhouse, “downstairs” is the domain of staffers such as Hudson (Gordon Jackson), the authoritarian butler.

The show began in the Edwardian era and spanned nearly three decades, with characters fighting in World War I, drowning on the Titanic and losing their fortunes in the 1929 stock market crash. For her part, Ms. Pagett’s character becomes involved with a group of socialist poets — marrying one of them — and joins the women’s suffrage movement.

Airing on ITV in Britain and on public television’s “Masterpiece Theatre” in the United States, “Upstairs, Downstairs,” attracted about 11 million American viewers a week, earned seven Emmy Awards and influenced period dramas such as Robert Altman’s “Gosford Park” and the television series “Downton Abbey.”

Ms. Pagett reveled in the show’s success — “There’s nothing more gratifying than busting into people’s homes,” she later joked — but left after two seasons, fearing she would be typecast by the role. The writers dispatched her character to New York.

She later appeared in the television movie “Frankenstein: The True Story” (1973), opposite Leonard Whiting and James Mason, and starred in the BBC’s 10-episode adaptation of “Anna Karenina” (1977), taking a role that had previously been played on-screen by Leigh and Greta Garbo.

Praising Ms. Pagett’s performance, a Time magazine reviewer called her “a breathtaking star” with “a face of strange beauty” that seemed perfectly suited to Tolstoy’s character. As Ms. Pagett put it in an interview with the New York Times, distinguishing herself from earlier Annas: “There’s nothing remotely ethereal or delicate about me. I’m sort of peasant stock. Words won’t blow me off my feet. I’m not fragile — not that sort of lady.”

Ms. Pagett was certainly not “fragile,” a term that was often used to stigmatize people with mental health issues. Two decades later, at a time when few celebrities or politicians spoke openly about mental health, she published a frank, lyrical memoir detailing her battle with manic depression, now known as bipolar disorder.

As Ms. Pagett told it, she was working on a National Theatre production of Joe Orton’s “What the Butler Saw” in 1995 when she had “a crack-up, breakdown, burnout — call it what you will.” She became infatuated with a public figure, whom she nicknamed “The Stranger” (he was later identified as Alastair Campbell, spokesman for Labour leader Tony Blair), and began writing him rambling letters. One included a check for 6 billion pounds, signed “Moi.”

In the grip of psychosis, Ms. Pagett accused her husband of incest and feeding their daughter heroin. She went to a psychiatric hospital three times, she said, before beginning to manage her condition with help from lithium, a prescription medication.

“Sometimes when I think about where I went, my breath gets caught,” she wrote in her memoir, “Diamonds Behind My Eyes” (1997). “But now, when I look at people, and I can tell they’ve been there and back, I feel quite proud. I’ve got a tale too, I want to whisper.”

Ms. Pagett noted that she risked losing future parts by discussing bipolar disorder, and soon stopped acting altogether. But she said she thought she needed to tell her story, as few celebrities seemed interested in addressing mental health concerns, and was pleased with what she had accomplished in getting treatment.

“I’ve never respected myself until now,” she told the Guardian, shortly before her book came out. “If I can deal with this, it’s like shaking hands with myself.”

Nicola Mary Paget Scott was born in Cairo, where her father worked as a Shell oil executive, on June 15, 1945. (After graduating from high school she changed her last name to Pagett — with two Ts, although only one was on her birth certificate.) Her father’s job led the family to move to Cyprus, Hong Kong and eventually Japan, where Ms. Pagett attended a Yokohama convent school and began acting at age 8, playing Snow White.

She later studied at an English boarding school in Bexhill-on-Sea before successfully auditioning for the Royal Academy of Dramatic Art in London. By age 19, she was performing in a West Sussex repertory company, acting in a different play each week, with two days to learn a part.

Ms. Pagett seemed on the verge of a professional breakthrough when she joined a 1965 touring production of “La Contessa,” a Paul Osborn play that starred Leigh but never made it to the West End. She later said she developed an eating disorder — another taboo subject for the times — while working on the play, which persisted until the birth of her daughter.

“There was a general consensus that I was a bit chubby and had to lose weight,” she told the Daily Mail in 1995. “I was only a size 10, but it was the Sixties and Twiggy reigned supreme. . . . I thought I would be worth more if I was thinner. Even when I appeared in ‘Anna Karenina’ on TV I thought I was too large. You wake up feeling fat although you’re not.”

Ms. Pagett made her London stage debut in “A Boston Story” (1968), adapted by Ronald Gow from a novel by Henry James, and launched her film career with movies such as “Anne of the Thousand Days” (1969), a period drama, and “There’s a Girl in My Soup” (1970), a romantic comedy with Peter Sellers and Goldie Hawn.

She appeared in the 1985 miniseries “A Woman of Substance,” about the makings of a business empire, and starred as the sexually promiscuous Liz Rodenhurst in the 1989 series “A Bit of a Do.” Her other screen credits include the marital sitcom “Ain’t Misbehavin’ ” (1994-95) and the coming-of-age film “An Awfully Big Adventure” (1995), starring Hugh Grant and Alan Rickman.

Ms. Pagett also continued acting onstage, notably starring in a 1985 revival of Pinter’s love-triangle play “Old Times” alongside Liv Ullmann and the playwright himself. Pinter played Ms. Pagett’s possessive husband, two years after directing her when she played Helen of Troy in a National Theatre production of “The Trojan War Will Not Take Place.”

In 1975 she married Graham Swannell, an actor turned playwright. They divorced in the late 1990s. In addition to her daughter, of Twickenham, England, survivors include a sister.

Ms. Pagett once said she began to gain confidence as an actress after performing at the Citizens Theatre in Glasgow, where she appeared in major works by Shakespeare and Racine. At the time, her dream was to perform on the West End.

“I wanted to go to London and act there — and I have, and I love it. But I don’t love it for the reasons I thought I would,” she told the Independent in 1992. “It doesn’t make me feel important, it doesn’t make me feel successful. I adore being in the paper and I love people knowing who I am, especially if they’re nice to me in the supermarket but, more than anything, I like looking into the eyes of someone whose work I respect and seeing them look back as if to say ‘I think you can do it, too.’ If anything means anything, that does.”

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