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Prince William’s girlfriend Kate Middleton a patient princess-to-be

(AP)

She’s not only the girl who would be queen — when she marries Prince William, Kate Middleton will be the first non-blueblood since the 1600s to marry a future British monarch.

She’ll also be the first to have graduated college, modeled lingerie and lived with her future king out of wedlock.

While rumors of an imminent en gagement have generated frenzied tabloid coverage in Britain and America, Middleton herself re mains a mystery.

Who is Kate Middleton? She’s a modern young woman who’ll make a traditional royal bride. Now 28, she met Prince William — he’ll turn 28 in June, the age he once said he’d consider getting married — when both were students at the University of St. Andrews in Scot land. The two have been together for seven years.

PHOTOS: KATE MIDDLETON

Her maturity, discretion and love for William have never been in question. She’s a media- savvy girl who’s never given an interview, re freshes her makeup before leaving a nightclub at 3 a.m., and has never been photographed stumbling out of a bar. She prefers to be called Catherine.

But to some in the young prince’s circle, Kate Middleton isn’t good enough to be the future queen of England.

After the couple broke up briefly in 2007, reports surfaced that some of William’s friends began mocking Middleton’s mother, Carole, calling the former airline stewardess “doors-to-manual” (a reference to landing procedure) and spreading rumors that she offended the queen by using the word “toilet” instead of “lavatory.”

James Whitaker, considered Britain’s top royal correspondent, said at the time, “I just don’t think she had the breeding, quite hon estly, and I’m not being snooty.”

William, by all accounts, was ap palled. He called Kate and her mother and apologized for the attacks.

“To suggest that the Palace was snobbish in any way about Miss Middleton and her family could be very damaging,” a senior aide told The Daily Mail. “That’s just not the way things work nowadays.”

THAT a commoner will likely be come queen is “on one level, extraordinary,” says Claudia Joseph, author of the new book, “Kate: Princess in Waiting.” “On another, it’s unsurprising. You can see it as defining an age when people are upwardly mobile, and Kate is typical of that.”

Middleton’s parents are from modest backgrounds. Her father, Michael, grew up middle-class and worked as an airline pilot; her mother, Carole, comes from a long line of coal miners and was a stewardess when she married Kate’s father in 1980. They launched Party Pieces, a direct-mail company specializing in party supplies, in 1987, and have since made millions.

Their fortune enabled them to send Kate (along with younger siblings Pippa and James) to Britain’s top schools. Middleton was educated at Marlborough, one of England’s most elite institutions, where she mixed with aristocrats and royalty.

Unlike her peers, however, Middleton never misbehaved: She didn’t drink or smoke, and she didn’t really date, either. Her friends teased her, calling her “Princess-in-Waiting.”

“She is very good looking and a lot of the boys liked her,” one of her classmates told The Mail on Sunday. “But it just used to go over her head. She wasn’t really interested, and she had very high morals.”

And like Diana, a poor student who, as princess, once said, “I’m a bit thick,” Middleton was called not “the brightest button” by another high-school classmate. But she was popular and had a self-deprecating sense of humor. A note scrawled by a pal in Middleton’s 2000 yearbook said: “Catherine’s perfect looks are renowned but her obsessions with her t- -s are not. She is often found squinting down her top screaming: ‘They’re growing!’ ”

Middleton and William met during their first year at St. Andrews; they lived on different floors of the same dorm, and became casual acquaintances, sharing an enthusiasm for sports and the outdoors. No one knows exactly when the two began dating, but chatter began in September 2002, after Middleton moved into an off-campus house with William and two other close friends.

In March 2004, they kissed in full view of the paparazzi while on vacation with Prince Charles and William’s younger brother, Prince Harry — without saying a word, they’d announced they were a couple and that Middleton met with the royal family’s approval.

“Kate has been brought up to eat with the right knife and fork,” says biographer Joseph, adding that all Buckingham Palace wants in a prospective spouse is “etiquette and style. I don’t think it’s about the right background.”

Commoners are those with no relation or connection to nobility nor royalty. Traditionally, they weren’t even good enough to have as mistresses. When the future King James II married the last commoner, 23-year-old Anne Hyde in 1660, he did it in secret, in the middle of the night. The nation was outraged. She gave birth to two future queens, Mary and Anne, before her early death in 1671. One of her death notices doubled as an appreciation for so ably manipulating a future king into marriage with a poor commoner.

Modern royalty has no such concerns. Spain’s Crown Prince Felipe married a former journalist. The crown prince of Norway married a single mother; the father of her child had been convicted on several drug charges.

“William’s not doing anything unique by choosing a commoner,” says Marilyn Braun, who runs a blog called The Kate Middleton Report. “It’s about who he loves, and that’s that. People are rooting for him to be happy, especially after Charles and Diana’s marriage.”

MOST royal watchers agree that the spectacular im plosion of that union largely accounts for the shift in the Palace’s attitudes towards future spouses. When Diana Spencer met Charles, she was, on paper, fit for a king: a 20-year-old blueblood with no past, a virgin who worked as a kindergarten teacher, a shy beauty who seemed to know her place. When Charles, then 32, and Diana announced their engagement in 1981, they were asked by a journalist if they were in love.

“Of course!” replied Diana.

“Whatever ‘in love’ means,” Charles added.

They divorced in 1996, and Charles publicly resumed his relationship with his current wife, Camilla, whom he’d fallen in love with as a young man but was deemed by the Palace, in the 1970s, an unsuitable wife for a future king.

Charles and Di’s very public, very ugly divorce, say royal observers, accounts for William’s reluctance to actually marry Middleton, though there is little doubt he’ll do it — he’ll just put it off for as long as possible.

When they are married, Kate will become Her Royal Highness Princess William of Wales (women take their husband’s full title), but she will most likely be called Princess Catherine. When William becomes king — most royal experts believe that Charles still wants “the top job,” as it’s called, so William may not ascend to the throne until he’s in his 50s — Kate will become Queen Catherine.

With her youth, beauty and class, Middleton is most likely to reignite international interest in the royal family, in steady decline since Diana’s death in 1997. She has, however, also been dubbed “Waity Katie” by the tabloid press for her aversion to work and her years-long availability to William.

Since graduating St. Andrews in 2007, Middleton has barely held a job. She was hired as an accessories buyer with clothing chain Jigsaw that November, but took the position with the caveat that she could leave whenever she wanted, for however long she wanted, whenever William called. She quit less than a year later.

What is clear is that Kate Middleton, an otherwise modern girl, lives in suspended animation until she and William announce their engagement.

That said, William and Kate seem far more suited for each other than Charles and Diana: Both are athletic and love the outdoors, both are homebodies, and both have very controlled, contained personalities. And Kate gives William — who was raised by his mother to appreciate more earthbound pleasures, like Disneyland and McDonald’s — something else he’s often said he wants: to be treated like everyone else.

“Kate and her family,” says royal blogger Ella Kay, “give him the opportunity to be normal.”

maureen.callahan@nypost.com