To draw inspiration for their fall collections unveiled this week, American fashion designers closed their eyes and thought of England.

In particular, many of them seemed to be thinking of Kate Middleton, the fiancée of Prince William. At the end of last year, just as designers were generating their ideas, Prince William proposed to his longtime girlfriend, generating a barrage of news on her sartorial decisions. Now, her brand of modern modesty—ladylike clothes that are also sexy and streamlined—is showing up as a fall theme at New York Fashion Week.

Other British-inspired styles on the runways included broad-brimmed hats at Tommy Hilfiger, the tiny headpieces known as fascinators at Marc Jacobs, and a surfeit of demure, pleated, calf-grazing skirts practically everywhere. "I think there's a little bit of royal envy going on here," said Robert Burke, president of Robert Burke Associates, a New York-based luxury-goods consulting firm.

On Tuesday, Vera Wang's show paid tribute to Victorian women with long, chiffon, knife-pleated skirts and high, gathered necklines, some accessorized with detachable fox-fur hoods and leather gloves. Models emerged from iron gates that resembled those of Buckingham Palace and walked to the soundtrack of "Atonement," a book-turned-film set in England between the wars.

The collection was meant to evoke "the grace, romance and effortless style of iconic American women who fascinated the English aristocracy of the 1930s," such as Wallis Simpson and Emerald Cunard, Ms. Wang said in the show's notes.

The focus on less revealing looks comes as designers are noticing an uptick in business, particularly at the high end. Women are showing a willingness to splurge on items with longevity. From a commercial perspective, Ms. Middleton's aesthetic is desirable, "because what she wears is...stylish, slightly conservative and classic," Mr. Burke said.

Designer Rachel Roy, known for her polished styles, unveiled a global-chic collection on Tuesday morning that took viewers to London by way of Guatemala. Ms. Roy showed her talents at mixing prints and lamé and added a chic capsule collection of eight trench coats and trench-coat-inspired looks that the designer said are "part menswear, but the wrapping is so feminine, so flattering, so pulled together."

Neiman Marcus's fashion director, Ken Downing, called the collection "ladylike chic with a bit of wanderlust."

Ms. Roy, who designed the gray frock worn by Michelle Obama to the State of the Union Address last month, said that she finds Ms. Middleton's style to be a breath of fresh air. "I think the effortless ease, the quiet, confident glamour she is bringing is so modern," she said. The blue dress that Ms. Middleton wore when she and the prince announced their engagement "was very age-appropriate but showed there was a fresh energy there," said Ms. Roy, whose fall collection contained a ladylike silk dress in a shade the designer called "blueberry."

Another designer who seemed to be channeling Ms. Middleton as a muse was Peter Som, who closed his show with a sexy-yet-modest royal blue sequin dress that would look smashing on the future royal.

Marc Jacobs's collection, shown on Monday evening at the Lexington Avenue Armory, was another homage to ladylike clothes. There was a mod series of polka-dot dresses and tops—some with prints, others with dot-shaped scales that resembled hanging chads. A series of Victorian-esque dresses featured high collars and ruffles at the neck. There wasn't a look in the collection that couldn't be worn to high tea at Claridge's in Mayfair—even if it was tea with Twiggy in 1967.

Kate and Laura Mulleavy, the sisters behind the popular Rodarte label, evoked a windswept prairie for their fall 2011 collection on Tuesday, which featured music by Pink Floyd. Long, sweeping dresses with high collars and calf-length coats created a scene in which an interwar intellectual like Virginia Woolf would have felt right at home.

And there are few who stand to benefit from Kate-mania more than Victoria Beckham, the British pop star-turned-designer who showed a collection of calf-skimming pleated dresses on Sunday in bright hues like saffron. After her show, Ms. Beckham said that Ms. Middleton has asked to see some samples.

"Kate Middleton is having a lot of influence on this whole Town and Country vibe, especially with all the pleated skirts with a kilt look," said Neiman's Mr. Downing. "I am calling it a 'Kilty Pleasure.' "

—Christina Binkley contributed to this article.