New York- A steamy bicoastal love affair between a multi-millionaire married grandfather and a sexy Manhattan socialite has spawned one of the most vicious and unusual child-custody disputes in the city’s history.

The courtroom battle, waged in secret until now, is over Amber and Scarlet Aylsworth, 4-year-old identical twins who were born out of an affair between John Aylsworth, 54, of California, who cheated on his wife, and Bridget Marks, 37, a former Playboy pinup, B-movie actress and romance novelist.

Aylsworth, who runs a casino gambling company, is demanding the twins be taken from their mother, who has reared them since birth in her East Side apartment.

“I am a mother who is trying to keep her children,” Marks said. “Every time I went to court I was slapped down. It’s been abysmal. It’s like functioning in another dimension where reality is immaterial. If they would give me my children, I would walk through fire. He won’t settle for anything but full custody. He hates me.”

Aylsworth wants the girls moved to his palatial seaside estate in Malibu, Calif., where he lives with Karen Aylsworth, his wife of more than 30 years.

She supports his demands even though she testified that she was devastated when she learned her husband’s lover was pregnant, and pressed for an abortion.

Legal experts said such a ruling, due to be handed down April 21, would be unprecedented.

“I’ve never heard of a case like this,” said Hal Mayerson, chairman of the Family Law committee of the New York City Bar Association and a veteran divorce lawyer.

“If they allow the children to be taken to California, they will effectively remove this mother from the children’s lives,” Mayerson said. “That should only be done under the most dire circumstances.”

But so far, John Aylsworth is winning Aylsworth vs. Marks, a saga now playing out in Manhattan Family Court before Justice Arlene Goldberg.

Both a court-appointed law guardian – a lawyer chosen to represent the children’s interests – and a court-appointed psychiatrist have urged the judge to take the children from their mother.

Aylsworth refused to discuss his custody claim. “We have no comment,” he said. “We think a media story is totally inappropriate.”

Marks, in tears at times, said in an interview that the experts are biased against her.

“Obviously, my children want to live with me,” she said. “They have their friends and their family and their toys. This is not the case where there’s the breakup of a marital home. This is the only home they have ever known.”

Marks, a Smith College grad with a master’s in international relations from New York University, appeared nude on horseback in a 1992 Playboy feature on New York City debutantes.

A stunning redhead, she has appeared in “Thinner,” a movie adaptation of a Stephen King novel, and “The Kings of Brooklyn,” a never-released film. Her father, Alvin Marks, 93, a Massachusetts physicist who was science adviser to President Kennedy, told Playboy that his daughter was “my best invention.”

Marks’ first novel, “September,” a 9/11 love story, is due in bookstores in July.

Aylsworth, a certified public accountant who amassed a fortune in the oil business, is now president and chief operating officer of a gambling operation, President Casinos, which includes a St. Louis, Mo., riverboat and a Biloxi, Miss., beach resort.

Aylsworth’s company went bankrupt in 2000 after defaulting on $100 million worth of bonds.

Asked why he is fighting so hard to take away children from an extramarital affair, Aylsworth said, “The reason is well documented through court records.”

Those records detail Marks’ allegations that Aylsworth has been grossly inappropriate in his behavior with the twins and countercharges that Marks is an unfit mother.

Aylsworth, who spends much of his time in St. Louis, contends Marks is alienating the children against him.

Marks has resigned herself to let him visit his daughters in New York.

Aylsworth refused to acknowledge paternity when he and his wife showed up at the hospital the day after Amber and Scarlet were born, according to testimony.

A few months earlier, Aylsworth and his wife told Marks to abort the children they now want, testimony shows.

“I was beyond devastated and very pro-choice, and I told Bridget that I thought Bridget should have an abortion,” Karen Aylsworth testified in court.

Marks said, “I didn’t know he was married when we first met because he doesn’t wear a wedding ring. “We never lived together,” she said.

The custody fight has cost both sides more than $1.3 million. Marks said she’s broke and owes lawyers some $700,000.

The big bills include fees for court-appointed experts, such as a psychiatrist who charged $42,500, and social workers hired at $200 an hour to supervise visits in New York and Malibu.

What surprised Marks was the behavior of law guardians appointed to represent the interests of her twins. The judge gave that job to Lawyers for Children, a public-interest law firm paid by the court to represent impoverished foster children and child-abuse victims in Family Court.

The firm sent its lawyer to Malibu to spend time with the father and children.

In writing, Lawyers for Children recommended custody go to Aylsworth, provided he and his wife move to New York. Then, the firm’s lawyer, Molly Murphy, changed her recommendation in the judge’s chambers to say the children should be moved to California, Marks said.

Officials at Lawyers for Children declined comment.

This week, Aylsworth asked the judge to issue a gag order to prevent Marks from discussing the case. His attorney asked to eject visitors from a public courtroom as the gag order was debated.

The judge refused to eject the visitors and is considering the gag order.

Meanwhile, Amber and Scarlet giggle and play with their mother, oblivious to the ugly fight over their home. They said they love their private preschool. An exclusive private kindergarten has accepted them for the fall.

After a recent snowstorm, the twins put on their matching pink parkas and skipped off to go ice skating at Wollman Rink in Central Park.

“It’s just breaking my heart,” Marks said. “I don’t know what I would do if I lost them.”

They met in 1998 at the Manhattan dinner party of a mutual lawyer friend.

John Aylsworth was the rich fiftysomething playboy casino operator. Bridget Marks was the thirtysomething former Playboy pin-up girl.

A torrid clandestine romance followed, with trysts at the Plaza hotel and other love-making landmarks.

“She thought he was wonderful,” said Molly Bennett Aitken, Marks’ mother, who owns the 83-acre Green Gables Farm in Athol, Mass.

“He asked her to marry him,” Aitken said. “His wife was living with another man – they had an open marriage.”

Aylsworth gave Marks a $7,000 diamond ring, and the couple took many trips together, including to Los Angeles and St. Louis.

Not long into their romance, Marks became pregnant.

“I was shocked, traumatized,” Aylsworth recalled in court. “I take full responsibility for the pregnancy for these children. She led me to believe she was on birth-control pills when, in fact, she wasn’t.”

Even after Marks gave birth to twin girls, her sexual liaisons with Aylsworth continued sporadically.

During this period, Aylsworth met Aitken at her daughter’s East Side apartment. He emerged naked from the bedroom to say hello.

“I said, ‘Would you please put some clothes on?'” Aitken recalled. “He said, ‘There’s nothing wrong with the human body.’ I thought at that point he was off the wall.”

In court, Aylsworth said he had no plans to marry Marks.

“I knew I was not going to marry her,” he testified. “I, you know, getting pregnant and having children and then subsequently making a decision about marriage are two different things.”

Forensic psychiatrist Dr. Stephen Bates Billick charged $400 an hour to evaluate Aylsworth, Marks and the twin girls and make a recommendation. He also billed $5,000 a day to testify in court and $10,400 to write his report.

Marks had agreed to bear half the cost of Billick and was told to expect to pay $2,000 or $3,000.

When her debt to Billick reached $14,000, Marks balked, and Judge Goldberg took a hard line.

According to court transcripts, Marks pleaded, “I have no money, so I don’t know what to do. I have to feed my children.”

“If it’s not paid, then there could be contempt,” Goldberg responded.

Later, the judge suggested Marks pawn her engagement ring from her fiancé, a high-ranking executive at Citigroup. “She has a $35,000 ring,” Goldberg said. “So there’s property at the very least from which she can obtain the funds.”

Marks, who was being helped out financially by her mother, asked Billick several times for an itemized bill.

“I haven’t typed it up, but I will type it up for you, the phone calls and everything,” Billick replied, according to a tape-recorded conversation.

“He never provided it,” Marks said.

On the witness stand, Billick called Marks narcissistic and delusional. Marks’ mother and fiancé helped her hire three experts to rebut his testimony.