He says she lives in a fantasy world.

And that's the starting point for what could be one of New York's most bitterly contested matrimonial cases this year - and they're not even married!

Koo Stark, ex-girlfriend of Britain's Prince Andrew and a former actress, wants more than $50 million from prominent Boston-born banker Warren Walker because he failed to marry her.

Stark is no pushover. She was previously married to Tim Jefferies, an English playboy and heir to a $100 million fortune, who is dating Claudia Schiffer and other supermodels.

Koo walked away from that marriage with a healthy settlement - in part because she claimed Jefferies was just an obscure rich guy until she introduced him to her blue-blood Brit friends and made him a celebrity.

She's also won more than $1.5 million in damages from London's tabloid press.

IN HER latest suit, Stark, now a world renowned photographer, alleges breach of promise and says Walker failed to provide for her and their three-year-old daughter Tatiana.

She wants $3 million in child support, a $1 million trust fund for Tatiana, a $4.8 million home, $20 million for breach of contract, $10 million for emotional distress and $20 million for loss of earnings caused by Walker's insistence she stop work.

Walker's lawyer, Phil Bronstein is planning to apply for a dismissal of Stark's case tomorrow. He claims Stark is not really a New Yorker, that she moved to Manhattan recently - and then, only for the purpose of filing her suit in a city that is used to handing out whopping settlements.

Bronstein also claims New York courts have never recognized promises of marriage as something that can be covered by contract law.

"There was no breach of contract, because there was no contract," he said.

Bronstein can expect a tough fight, and not just because Stark is six-for-six in court battles.

The sultry snapper has hired famed divorce lawyer Raoul Felder - currently biting Rolling Stone Mick Jagger in his infamous child-support suit - who claims the former Royal squeeze is a New Yorker to her bones.

Felder believes Stark will win plenty of sympathy from a Manhattan jury.

THE once nearly-royal Koo says her relationship with Walker was gruesome. She says the multimillionaire is a borderline sociopath who tormented her with equal doses of affection and hatred.

She believes Walker wanted to use her connections to make Britain's royals, the Astor family and dozens of other celebrities, his banking clients.

The couple met in January 1995, when Koo was selling her apartment near London's famed Harrods department store. Walker decided to take the place and all its contents - including Koo.

After pursuing her for six months, Walker wanted her to sleep with him. She said that could only happen if he promised to be a faithful and loving partner. Walker made the promise and the couple became passionate lovers.

Toward the end of 1995, Koo and Warren, both Buddhists, spent time in Mongolia with the Dalai Lama.

It was the beginning of their troubles.

KOO said she began to realize how reckless Walker could be after he set off on a solo horseback trip into uncharted hill country. He got lost and almost died of hypothermia. Koo said the Mongolia incident set the pattern for their relationship. She claims Walker would give her intense affection only to suddenly disappear unexpectedly and become unobtainable.

A classic example occurred during Christmas, 1995, she said.

Walker is from an old New England family. His father is an architect; his mother a Boston socialite. Koo claims he pressed her to make introductions for him with people he thought would impress his family.

During summer, 1995, she took him to meet old money billionaire John Astor at Hatley, his English country estate.

The Astors invited the couple back for Christmas but Walker failed to show up. Koo said she was heartbroken and sought comfort from her friends, including Prince Andrew and his former wife Sarah Ferguson.

They all begged her to leave Walker. Koo was on the verge of breaking up with the banker when he popped back into the picture with flowers, gifts and apologies.

He wanted her back, Stark said, and he whisked her off to Tanzania for a two-week safari where the couple's noisy and passionate love-making became a subject of comment among fellow travelers.

FROM Tanzania, the couple went to Boston for the 1996 summer wedding of Walker's cousin. Walker proposed. Koo was delighted - and, even more so a month later, when she discovered she was pregnant.

But then, unexpectedly, Walker demanded she have an abortion.

He allegedly declared he "couldn't possible have a child out of wedlock, since that kind of thing just wasn't done in his family."

Koo said she was stunned by his sudden display of Victorian morality and offered to get married immediately.

The suddenly pious Walker - who previously had not been above fornicating at maximum volume - protested a quick marriage would "look like a shotgun wedding and create a family scandal."

Koo was familiar with family scandals, having caused one in the British Royal Family when her relationship with Prince Andrew, then a bachelor, was revealed.

Andrew, who later went on to a disastrous marriage with Sarah Ferguson, was clearly smitten but was forbidden ever to even think about marrying Koo by the Queen herself.

Koo said Walker seemed to be trying to create problems in the relationship. She soon discovered why.

PACKING his clothes for a climbing expedition, Stark, found an engagement ring and a ÔDear John' letter from one of his former girlfriends.

The letter made clear Walker had spent Christmas 1995 - when he should have been with the Astors - with this woman. He had even been engaged to her when he first began talking about marriage with Koo.

When the engagement broke off, Walker came running back to Stark claiming he had spent the time away seeing a psychiatrist and taking Prozac.

"You see a pattern in Walker of somebody who is a bit unbalanced and vicious with it," said one of Stark friends.

"He used her and tortured her with a mix of love and lies. I think he wanted the abortion because he still had hopes of getting back with this other woman."

Stunned by Walker's deceit and his demands for an abortion, Koo told Walker she would have the baby with or without him and left for England.

SUMMER 1996 shaded into fall and Walker was nowhere to be seen. Suddenly, Koo alleges, Walker appeared on her doorstep with profuse apologies and a new offer of marriage.

Koo said he now promised to support her and the child with generous payments and the purchase of a $3 million house in a fashionable part of London.

Their child was born in Boston, on March 17, 1997.

Walker was at the hospital for the birth but allegedly disappeared within hours on an unplanned three-day trip. On his return, he behaved like nothing had happened and again showered Koo with presents.

Again, Koo's friends urged her to get away from Walker and find a different life. Koo says she first began thinking along the same lines in September 1997, when she discovered she was pregnant again.

WALKER vehemently denied the child could be his, Stark said, and alleged it had been fathered by one of Koo's former men. He named Prince Andrew as one of the possible candidates.

Koo says Walker's denial of a child she knew to be his threw her into deep depression and caused abdominal pains and bleeding.

Four months into her second pregnancy she was advised the baby was almost certain to be stillborn. With great reluctance she decided on an abortion.

True to form, Walker reappeared the day after the termination, paid the medical bills then left immediately on his own for a four-week trip to Africa. The abortion destroyed any love Koo had left for Walker, she said. She spent time with him thereafter only for the sake of Tatiana.

Aware that Walker might not keep his promises, Koo began an aggressive campaign to accelerate her successful photography career. But her growing financial independence seemingly angered Walker.

He now allegedly demanded that, in return for him paying substantial sums for Tatiana and Koo's upkeep, she help entertain his clients.

Whenever Koo was offered a big photo shoot, or TV work, Walker tried to block it. He said her "real job" was at home caring for Tatiana.

Koo says her success was threatening to Walker. She believes he had been using his money to control her and, when that failed, he began to abuse and intimidate her and her daughter.

STARK paints a picture of an insecure man who wanted a wife he could control rather than a partner as strong as himself and who was a social climber entranced by her access to royalty.

"If it had just been me, I could have forgiven and forgotten all this and moved on," Stark said last week through her lawyer.

"I am following the painful course of this lawsuit solely to ensure security for Tatiana."

Walker's lawyer, Phil Bronstein, has nothing but contempt for Stark's suit.

He dismissed her claims as "fantasy" and said he believes New York courts will make quick work of the woman who once had Buckingham Palace quaking with fear.

"We have a reputation for legal insanity in Manhattan divorce courts," says Bronstein. "But this suit takes madness to a whole new level.

"If she wins and gets the bulk of Mr. Warren's fortune the message will be no man in New York can back out of a marriage, however sensible that decision might seem, without risking financial ruin."