For New York’s young scions, it is a nightmare scenario: A wealthy, widowed father marries a much-younger former model. He dismisses the idea of a pre-nuptial agreement. They divorce. He’s then forced to pay her hundreds of millions of dollars – cutting deep into the heirs’ inheritance.

Four adult children of a former Beatle, Paul McCartney, may now face this predicament following reports their rock-idol father is splitting from his wife of four years, Heather Mills. Speculation about the terms of their expected divorce began almost immediately, because the British press has long suspected the couple did not have a pre-nup.

New York divorce lawyers estimated that Ms. Mills, 38, could walk from the marriage with more than $200 million – not including any court-ordered alimony or child-support for their 2-year-old daughter, Beatrice Milly.

“A substantial portion of his wealth will be transferred to Heather Mills,” a New York divorce lawyer who has worked with British couples, Susan Moss, predicted. “If he doesn’t make her an offer she can’t refuse, then they’re off to the races.”

Sir Paul has three grown children and a stepdaughter from his 29-year marriage to his late wife, Linda, who died of cancer in 1998. Among them is a New York designer, Stella McCartney, who reportedly disapproved of her father’s relationship with Ms. Mills.

High society prenuptial agreements – or lack thereof – have caused more than a few family feuds. The chairman of Revlon, Ronald Perelman, skipped his son Joshua’s wedding after the younger Mr. Perelman refused to sign a pre-nuptial agreement several years ago. The older Mr. Perelman is no stranger to the economics of marriage – he’s been married four times and is in the process of divorcing an actress, Ellen Barkin.

Prenuptial agreements are de rigueur among wealthy Americans but remain uncommon across the pond. Local laws generally dictate divorce settlements in Britain and spouses often receive half of what was earned during the marriage.

“In the U.K., there’s a feeling that prenuptial agreements are wrong, and shouldn’t be enforced,” Ms. Moss said.

Both Sir Paul and Ms. Mills are from England. The couple was married in Ireland in 2003.

Even if Ms. Mills receives a settlement worth several hundred million dollars, there should be plenty left over for Sir Paul’s heirs given that his vast fortune is said to be worth more than $1.5 billion – and growing.

Ms. Moss said she’s written prenuptial agreements for British couples and American expatriate couples living in England, who agree to abide by New York law if they divorce. “It’s not uncommon for U.K. citizens, especially if they have a second residence in New York, to call an American lawyer,” she said.

Another divorce attorney, Raoul Felder, who has represented Mayor Giuliani, said the vast majority of mega-millionaires and billionaires worldwide insist on prenuptial agreements. Mr. Felder was skeptical about the reports Sir Paul and Ms. Mills did not have one.

“He would need a psychiatrist, not a lawyer if he didn’t have a prenup,” he said. “He earned an enormous amount of money during that marriage, and her presence enhanced the value of his portfolio. She brought another dimension to his character.”

Children from a previous marriage are often the impetus for a prenuptial agreement, according to president of the Family and Divorce Mediation Council of Greater New York, Adam Berner.

“Very often, the primary commitment of a parent is to his children, and they want to protect them,” he said.

A recent Supreme Court case put the spotlight on how adult children of wealthy parents can react furiously when their inheritances are threatened. A son of a Texas billionaire, J. Howard Marshall, is trying to prevent a model Anna Nicole Smith, from taking a sizable portion of his late father’s estate. Mr. Marshall was 89 when he married Ms. Smith, who was 63 years younger.

The lawyer representing Mr. Marshall’s son, Eric Brunstad Jr., said heirs often suffer when inheritances wind up in court:

“When these things go to litigation, it’s never a happy story.”