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The 43-year-old beat Elle Macpherson, 44, in Good Housekeeping magazine's annual list.
She was praised for her "innate sense of style that never looks studied or overdone", while McPherson was branded "the absolute mistress of throwaway chic".
Other names on the list included TV style guru Trinny Woodall, 44, Sex And The City actress Kim Cattrall, 52, Michael Caine's wife Shakira, 61, and ex-Bond girl Honor Blackman, 80.
Last year's number one Sharon Osbourne did not feature.
Good Housekeeping fashion director Jane de Teliga said: "Why is it that, when Helen Mirren was snapped this summer on the beach sporting a red bikini, it sent shockwaves through the media? What was more startling - that she was over 60 or that she looked fabulous?
"In our youth-obsessed world, Good Housekeeping wants to fly the flag for women over 40 who look fantastic."
