One of the world's wealthiest women was in a Bermuda court yesterday, fighting a determined businessman's claim on her luxury island retreat.

American heiress Marion MacMillan, whose family is worth an estimated $5 billion, agreed to sell peaceful Perot's Island to Michael De Groote, former head of a top Canadian company.

That was in 1990. But Mrs. MacMillan changed her mind, and she is now being sued by Mr. De Groote.

The 59-year-old tycoon has cut his links with Canada and has chosen Perot's Island to recreate the luxury lifestyle he enjoyed back home. He wants the court to force Mrs. MacMillan to sell him the island.

But Mrs. MacMillan, also 59, has fought back. She has hired lawyer Mr. Julian Hall and hauled the Bank of Butterfield and real estate firm Cooper Associates into court, raising questions over their handling of the deal.

The 71 acre island in Riddell's Bay has a main house with around 15 rooms, including about five bedrooms and seven bathrooms. It has a beach, a swimming pool, and separate guest quarters. It comes with a half-acre plot on the mainland with an office and apartment.

Mrs. MacMillan inherited around $700 million after her father's death in 1960.

The family were 19th on last year's Forbes magazine list of the world's richest people. Cargill, the family's grain trading firm, is the largest privately-owned company in the US.

In 1987 Mrs. MacMillan bought Perot's Island for around $3 million. She originally planned a retreat along "New Age'' philosophical lines.

In 1990 Mr. De Groote agreed to buy it for $8.5 million, the Supreme Court heard yesterday.

His lawyer, Mr. John Riihiluoma, told of Mr. De Groote's rise from "humble origins'' to chief executive officer of Laidlaw, a big transport and waste disposal business.

He decided to leave Canada, partly to preserve his income, and was told about Perot's Island by Mrs. Elaine Darling of Cooper's, a family friend. Mr. De Groote reassured Mrs. MacMillan that he would respect and care for the island and would not build a condo complex, said Mr. Riihiluoma.

But Mrs. MacMillan backed out. To this day Mr. De Groote does not know why, said Mr. Riihiluoma.

He said faxes and letters showed clear roles for the agencies involved, with a clear "chain of information'' from Cooper's to the bank real estate company, to Mrs. MacMillan's lawyers and Mrs. MacMillan.

Mr. Riihiluoma said Mr. De Groote had given up a lot in leaving Canada, including his residency and his company posts. Buying Perot's Island had been a "very integral'' part of his plan.

GOING INTO BATTLE -- Multi-millionaire Mrs. Marion MacMillan enters Supreme Court with her lawyer Mr. Julian Hall, to fight to keep Perot's Island.