NEW YORK (AP) - Michelle Obama chose an off-the-shoulder Marchesa gown for her duties Wednesday night as hostess at a White House state dinner honoring the British prime minister and his wife, David and Samantha Cameron.

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It was a deep shade of teal in a draped, column silhouette.

The designer of the gown, Georgina Chapman, and her husband, movie mogul Harvey Weinstein, were guests at the affair, but they didn't know Mrs. Obama was wearing one of Chapman's designs when they arrived.

"I'm knocked out!" Weinstein said.

Chapman, not surprisingly, also wore Marchesa, which has made its name on Hollywood red carpets. Hers was a draped blue and gray gown with a sheer overlay, and covered in crystal embroidery.

Mrs. Cameron seemed to get the memo to wear blue too: Her sparkly gown by London-based designer Alessandra Rich featured lace details, a high neck, half sleeves and a belted waist. She had on high black heels from the British retailer Next.

There's great interest in what Mrs. Obama wears to big events like this because she's been touted as a fashion influencer who makes bold choices. On Wednesday, the unexpected piece of her look was the chunky, multistrand silver, turquoise and teal necklace by Tom Binns. Binns also had created the elaborate necklace the first lady wore last year in London when she and the president welcomed the British royals to the American ambassador's home for a black-tie dinner.

"First lady Mrs. Obama always looks so chic and modern. I love the addition of texture with the Tom Binns necklace," said Roksanda Ilincic, who has dressed both Mrs. Obama and Mrs. Cameron.

Mrs. Obama, in fact, wore a purple dress by Ilincic during that same trip to England.

"Dressing Mrs. Obama and Samantha Cameron have been wonderful. I am honored that each woman representing their countries have been such advocates in their respective roles," Ilincic said.

The other fashion plate at the dinner was Vogue editor-in-chief Anna Wintour, a well-known supporter of President Barack Obama. She wore Chanel.