Her death was reported on her Web site, and confirmed by her manager. She had been treated for cancer at a hospice in East Sussex, near her home in St. Leonards-on-Sea, in the south of England.
Ms. Said (pronounced sah-EED), the daughter of a Somali father and a British mother, who raised her alone, began performing as a free-spirited teenager, leaving home at 15 and ending up in London, where she studied to be an opera singer, said her manager, Shirin Koohyar. In 1976 she released a pop-reggae single under the name Mari Elliot on the GTO label.
But after stumbling upon an early performance by the Sex Pistols, she was inspired to place an ad in a British music magazine, searching for "young punx who want to stick it together."
After a handful of rehearsals, the X-Ray Spex whose early lineup included Jak Airport, Paul Dean, Rudi Thomson, BP Hurding and Lora Logic quickly became fixtures on the 1977 London music scene. They performed in Chelsea and at the Roxy, the Covent Garden club that served as a punk incubator, adding a new instrument, the saxophone, to the chopping guitars and brazen lyrics of the genre.
The original group released just one album, "Germ Free Adolescents," in 1978, but it became a punk classic, summing up the era's "sass and adrenaline," as Jon Pareles wrote in The New York Times in 2006, reviewing a box-set of the group's music.
X-Ray Spex toured abroad, playing in Los Angeles and at CBGB in New York, cross-pollinating with acts like Blondie and Richard Hell of the Voidoids. (For the album and the tour, Rudi Thomson replaced Lora Logic, born Susan Whitby and still a schoolgirl, on sax.) Ms. Said wrote lyrics questioning social mores, commercialism and conformism and, as a boldly-styled, biracial young band leader, lived up to them.
As a performer, Ms. Said was a rare combination of taunting and cheerful. Dressed in Day-Glo colors the album cover of "Germ-Free Adolescents" has the band in pink, acid green and bright yellow and bouncing unselfconsciously around the stage, she served as inspiration for other female musicians, prefiguring movements like riot grrrl.
"Some people think little girls should be seen and not heard," she shouted at the beginning of X-Ray Spex's best-known anthem, before delivering its punchy title and chorus: "I say, Oh bondage! Up yours!" The song, she said later, was inspired partly by Vivienne Westwood's 1970s bondage fashion, but it was her styling that made it stand out.
"Poly Styrene's voice on 'Oh Bondage! Up Yours!' was the most exhilarating voice I ever heard it was all body," said Kim Gordon of Sonic Youth.
Kathleen Hanna, the riot grrrl progenitor and leader of the bands Bikini Kill and Le Tigre, whose style is often linked to Ms. Said's, said she felt unworthy of the comparison. "Poly lit the way for me as a female singer who wanted to sing about ideas," Ms. Hanna wrote on her blog. She added, "Her lyrics influenced everyone I know who makes music." After the X-Ray Spex broke up in 1979, Ms. Said retreated from the spotlight, releasing music only sporadically, as she dealt with personal issues. She joined the Hare Krishna movement and later received a diagnosis of bipolar disorder.
Though a second X-Ray Spex record, "Conscious Consumer," released in 2005, was not well-received, two reunion concerts, in 1991 and 2008, drew widespread fan attention. After the second concert, Ms. Said began working on a solo album, "Generation Indigo," her first new work in seven years. It was released in March.
She is survived by her daughter, Celeste Bell-Dos Santos, who leads the Spanish band Debutant Disco; her mother, Joan Elliot; a brother; and a sister.
Though in a recent interview Ms. Said said she would like to be remembered for something "more spiritual" than her early punk efforts, she was also aware of the impact she had on music.
"It's worked out fine," she said in a 2008 interview with British newspaper The Independent. "I feel better for having been on stage, having been told I never could. I'm starting to think, maybe what I did then is working. Oh, I didn't waste my time. My youth wasn't misspent!"