Vivienne Westwood, the provocative British fashion designer who endured for more than half a century as a style icon and environmental activist, helping clothe the 1970s punk movement before dressing supermodels on the runways of Paris and Milan, died Dec. 29 in the Clapham section of southwest London. She was 81.

Her death was announced by her namesake fashion house, which did not cite a cause.

Ms. Westwood grew from an enfant terrible into a grand dame of the fashion world, bursting onto the London scene in the 1970s when she helped dress punk rockers like the Sex Pistols with leather jackets, ripped shirts and safety pins. She later moved into couture design, creating outfits that were exhibited in museums around the world, while experimenting with flamboyant pirate shirts and petticoats, tweed corsets and pinstripe tailoring.

Throughout her career she linked fashion to politics, leveraging her fame to promote environmental causes, nuclear disarmament, vegetarianism and efforts to fight climate change. She emblazoned her shirts, jackets and dresses with activist slogans — “Politicians R Criminals” and “We are not disposable” — and urged her audience to buy less, not more, unveiling a unisex line in 2017 with the hopes that men and women could share the same clothes, including capes and tutus.

“Unisex may sound like a joke, but, in fact, it’s all about styling and being able to dress however you like,” she told the New York Times that year at a London Fashion Week event. “Swapping clothes with your partner means you can buy less, choose well and really make them last.”

Ms. Westwood was initially known for presiding over a boutique on London’s King’s Road with Malcolm McLaren, who became the manager of the Sex Pistols. “I was messianic about punk,” she later recalled, “seeing if one could put a spoke in the system in some way.”

She maintained that anarchic sensibility long after she was embraced by the establishment, posing for the cover of Tatler magazine in 1989 in an Aquascutum suit that she said was intended for Prime Minister Margaret Thatcher. Three years later, when Queen Elizabeth II awarded her the Order of the British Empire, Ms. Westwood — who was later named a Dame Commander — shocked photographers by twirling to show off her outfit, a tailored skirt suit that she wore with sheer tights but no underwear.

“I have an in-built perversity,” she said in an interview for the book “England’s Dreaming” by Jon Savage, “a kind of in-built clock which reacts against anything orthodox.”

Vivienne Isabel Swire was born in Glossop, an English town east of Manchester, on April 8, 1941. Her mother, a seamstress who made her own clothes, favored standard fare for her three children; Ms. Westwood said that she began to dabble in fashion when her mother allowed her to pick out her own clothes. She selected a tight skirt and heels.

Ms. Westwood briefly attended Harrow Art School and then went to a teacher’s training college, landing a job as a schoolteacher. Her marriage to Derek Westwood, a dance hall manager, ended in divorce, and in the mid-1960s she began a relationship with McLaren, with whom she collaborated as a designer.

Together they riffed on the slim-tie, gelled-hair fashion style of the 1950s “teddy boys,” while also drawing on biker culture and sadomasochistic imagery. Under the slogan “Clothes for Heroes,” they designed leather-and-zipper dresses and “bondage” shirts with sleeves that wrapped around like a straitjacket. One T-shirt showed Queen Elizabeth II with a safety pin piercing her lip.

Their shop cycled through several names, including Too Young to Die and Too Fast to Live, but the one that most caught the public’s attention was Sex. The name appeared in bloated pink letters above the door.

In 1981, they launched their first runway collection: gender-neutral clothes that evoked pirate imagery and 19th-century fashion. The style became part of the post-punk New Wave scene after it was adopted by pop stars Adam Ant and Boy George.

Ms. Westwood soon dissolved her partnership with McLaren, and went on to create designs including the mini-crini, a shortened version of the Victorian crinoline, and a lightweight corset designed to be worn on the outside of an outfit, which helped spark a ’90s trend toward underwear as outerwear. In recent decades, she used her name for an expanding array of retail partnerships, including teas sets, hats, jewelry and fragrances.

She opened her first U.S. boutique in Los Angeles in 2011.

Survivors include her husband of 30 years, Austrian designer Andreas Kronthaler; a son from her first marriage, erotic photographer Ben Westwood; and a son with McLaren, Joseph Corré, who co-founded the lingerie brand Agent Provocateur. Additional details on survivors were not immediately available.

In 2008, a Westwood wedding dress became a centerpiece of the “Sex and the City” movie when Sarah Jessica Parker’s character, Carrie Bradshaw, decides against a Vera Wang dress in favor of Ms. Westwood’s billowy silk-and-taffeta. Ms. Westwood was unimpressed with the rest of the clothes featured in the film, later saying: “I went to the premiere and left after 10 minutes.”

When it came to her own image, she often opted for relatively uncomplicated outfits to go with her distinctive bright-orange hair. “My fashion advice,” she told the Times in 2009, “is to have a flattering mirror at home and then forget about it.”