The steely intelligence she brought to parts gave British actress Glenda Jackson, who has died aged 87, a reputation for being uncompromising and prickly, but it reflected a profound seriousness about her art. “I regard acting,” she once said, “as a serious job for serious-minded people.”

That manifested in a long career of intense and brilliant performances. Her acting was self-critical and unsentimental, driven, director Charles Marowitz said, by a “loathing for lame and easy effects.” Notwithstanding a reputation for asperity, her seriousness also gave her great strength with comic material.

Her critical thought meant she was always striving for emotional and intellectual clarity, whether in interpreting Shakespeare, filming with Ken Russell or singing sea shanties with the Muppets. The actor Jonathan Pryce said she was “always direct, always honest. And, like the greatest art, her work was simple and uncluttered.”

The decisive directness and critical faculty she brought to acting reflected in part her working class upbringing. Her mother Joan was a cleaner, her father Harry a bricklayer. She told the Times: “I come from a family where if you didn’t work, you didn’t eat. That was the class structure.”

Her political understanding of this was social democratic. Obituarist Michael Coveney said Jackson was “known for having concerns rather than ideas, and these were rooted in her background of working-class poverty, and her belief that the arts had both a higher purpose and a responsibility to educate and inform.”

She was a lifelong Labour Party member, and said she would never leave, but her instinctual universal decency and democracy inevitably found less reflection in a party lurching ever more rapidly to the right, no matter how loyally she defended it.

As an actress, however, her self-awareness was cultivated early. She described herself as having been “an archetypal spotty teenager who suffered the tortures of the damned because I wasn’t like those girls in the magazines.” Rather than hiding or changing this, she used it as the basis for exploring and displaying the most intense of inner experiences. Marowitz wrote of her “scrubbed raw as if to obliterate her features … a softly pulsating indictment of everything crude, crummy and unworthy in our work.” 

There were no childhood dreams of acting, but she felt the limitations of cultural life afforded the working class: “It came to me, later, that there was more to life than I was experiencing and I had more to offer than was being asked of me.”

After failing exams, she left school at 16 to work in Boots the Chemist. She set out to alleviate her boredom by joining an amateur dramatics group.

There was still no notion of a career. “I had no real ambition about acting,” she recalled, “But I knew there had to be something better than the bloody chemist’s shop.” When, after two years, somebody suggested, “You should do this professionally,” she “wrote to the only drama school I’d ever heard of, which was RADA [the Royal Academy of Dramatic Art],” auditioned, and was accepted.

RADA said she would have had a scholarship if they could have afforded one. Her manager at Boots wrote to Cheshire County Council, which gave her a discretionary grant to study. 

She arrived during a sea-change in British theatre. There was a shift away from the affected drawing-room drama which had dominated. Instead of plays about the affluent bourgeoisie in the drawing rooms of their country homes, or upper-middle-class layers in comfortable suburbia, dramatists were beginning to turn to the working class and the lower middle class, struggling in bedsits or terraces.

RADA, in turn, was changing, and beginning to train a different sort of actor. Jackson was part of a pioneering wave of students like Alan Bates and Peter O’Toole.

After RADA, there were lean periods as well as repertory theatre work, and an early film appearance in Lindsay Anderson’s This Sporting Life (1963). The Royal Shakespeare Company (RSC) turned her down. She struggled to make ends meet, but when work did pick up, her directness was appealing for directors at the cutting edge of experimentation in classical theatre, like Peter Brook, then assembling a company for his 1964 Theatre of Cruelty season as part of his reinvigoration of the RSC.

Marowitz, working with Brook, was a partisan champion of Jackson. His sometimes overblown reminiscences give a useful summary of Jackson’s qualities, and their background. Assembling Brook’s company was itself experimental. “The bourgeois members of the acting profession, who make up the majority,” wrote Marowitz, “caught wind of things very early and either stayed away or began to cast opprobrium” on the “unBritish and even exotic” work.

Jackson stayed. Marowitz wrote that some actors found her difficult to work with, but only because she “radiated disgust” at “the sloppy, ill-defined, unthought-out mugging which passed for acting in the British theatre and, particularly, the appearance of those characteristics in herself.

“Whenever she worked, one could hear her built-in bullshit detector, that most delicate of all precision instruments, ticking in the background, and the actors who resented her most were those whose execrable effusions were being scrutinized and judged in the glare of those cold, sleepy, cruel eyes.”

Her RSC seasons over the next two years were electrifying. David Warner gave the defining Hamlet of a generation, but he was thrillingly matched by Jackson’s Ophelia. In Peter Weiss’s The Investigation, Jackson and Penelope Keith played all the female witnesses at Auschwitz.

Even more significant was Brook’s production of Weiss’s Marat/Sade, in a version by radical poet Adrian Mitchell, with a remarkable cast including Patrick Magee and Ian Richardson. Jackson reprised her performance as Charlotte Corday in Brook’s film of the production, and also appeared in Brook and Mitchell’s US, a directly political response to the Vietnam war (filmed as Tell Me Lies).

Alongside towering stage appearances—including in William Gaskill’s acclaimed Royal Court production of Chekhov’s Three Sisters (1967), with Marianne Faithfull—Jackson was beginning to make equally impressive movies.

Almost her first major performance was as Gudrun in Ken Russell’s vision of D.H. Lawrence’s Women in Love (1969), with Bates and Oliver Reed. The performance brought her first Oscar and established a long mutual respect between Jackson and Russell.

To avoid being swamped by Russell’s exuberance required a robust strength from his actors, which suited Jackson perfectly. Reed called acting alongside her like “being run over by a Bedford truck,” while one critic said of her “powerhouse performance” in The Music Lovers (1970) that she was “one actress who can hold her own against Russell’s excess.” And, more to the point, ground them in reality.

Marowitz suggested that her extraordinary range was rooted in “her dry-as-dust background that inclines her toward performance choices which are exotic and hierarchical.” Yet it also informed an ability to make those performances believable and real.

By the early 1970s this was displayed in a wide variety of material, from adaptations of H.E. Bates with Michael Apted to John Schlesinger’s Sunday Bloody Sunday. Exemplifying all her qualities, Queen Elizabeth I became a defining role.

In the six-part BBC series Elizabeth R (1971), Jackson portrayed Elizabeth from 16 to 69. She simultaneously played Elizabeth in Charles Jarrott’s Mary, Queen of Scots, modelled on Friedrich Schiller’s fictional meeting between Elizabeth and her cousin Mary Stuart (Vanessa Redgrave). The film has an extraordinary cast (Timothy Dalton, Ian Holm, Trevor Howard), and the brilliant performances of Jackson and Redgrave—another product of the innovative RSC in the 1960s—offer a striking contrast in style and technique.

But Jackson’s seriousness allowed her to view herself wryly, too. That year, she appeared with the comic duo Morecambe and Wise, playing Cleopatra in a mock-awful “classical” play. Her timing was a revelation, and led to the offer of a romantic comedy, A Touch of Class (1973) with George Segal.

This brought her second Oscar, but it is to her credit that she had no interest in the ceremony. “You don’t do a play to compete for an award. I didn’t win them. They were given to me.” That was healthy.

She continued to be given awards, though, because the work continued to be exceptional. Some was filmed, like Jean Genet’s The Maids (1975) and Hedda (1975), Trevor Nunn’s production of Henrik Ibsen’s Hedda Gabler.

After her marriage broke up, there were a couple of less interesting Hollywood films, although she continued to find worthwhile theatrical productions. Some of these were also filmed, like Hugh Whitemore’s Stevie (1977), about the poet Stevie Smith. There were stage duds, too, including Brook’s misjudged Antony and Cleopatra (1978), but her theatrical instincts remained sound.

The Glasgow Citizens theatre under Giles Havergal and Philip Prowse was an inspiration, prompting some great work in the 1980s. She worked several times with former Citizens director Keith Hack, including in Eugene O’Neill’s Strange Interlude (1984), and appeared in a lauded production of Federico García Lorca’s The House of Bernarda Alba (1986). The pinnacle was Prowse’s production of Racine’s Phèdre at the Old Vic, described by Coveney as “the [Brook] Cleopatra that went missing.”

Jackson had felt driven to identify herself more with the Labour Party in opposition to the government of Margaret Thatcher (1979-1990). In 1992 (after Prowse’s blistering production of Brecht’s Mother Courage), she stood as Labour candidate for Hampstead and Highgate.

Then-Labour leader Neil Kinnock tried to dissuade her from standing for parliament, on the grounds that she was a great actor first and a Labour member second. We might echo that, but should credit her seriousness here, too. There was no dilettantism. She quit acting, winning the seat even as Labour were routed.

Her ensuing parliamentary career is a record of the evolution of the Labour Party, although she was increasingly at odds with the party, she would not leave. Reelected in 1997, she condemned the Iraq war, threatening to stand as a stalking horse leadership candidate in 2005 to force Blair’s retirement as prime minister. In 2013, Jackson won respect everywhere but parliament for her refusal to fawn over the deceased Thatcher.

To the howls of Tory MPs, she denounced the “heinous social, economic and spiritual damage” wrought by Thatcherism, where “everything I had been taught to regard as a vice—and I still regard them as vices—was, in fact, under Thatcherism, a virtue.”

The outrage came not just from Tories. Blairite hack Dan Hodges accused Labour critics of being “childish” and “self-indulgent” in their attacks on Thatcher. Hodges is Jackson’s son. She said the two of them were “implacable” in their politics.

It was theatre’s gain when she stepped down from parliament in 2015. We may regret 25 years when she was not performing, but her return was triumphant and driven by the same impulses. In 2016, she gave an earth-shattering performance as King Lear, aged 80. She reprised the part in New York four years later, having won a Tony for Edward Albee’s Three Tall Women the year before. To the end there were powerful screen performances.

Jackson was never cosy or frivolous as a performer, even in light material. Every role was treated with the seriousness and consideration it demands. There is a lot to learn from her acting.