Katrin Radmacher has won a landmark Court of Appeal case to enforce a pre-nuptial deal that protects her personal wealth from legal claims by her former investment banker husband.
The German heiress worth £100 million, succeeded in her challenge to a court ruling that she pay her husband more than £5.5 million.
Lord Justice Mathew Thorpe ruled Ms Radmacher's French husband, Nicolas Granatino, is to be awarded £1 million plus another £2.2 million-pound loan for a home that will have to be returned when the couple's two daughters grow up.
The couple made the pre-nuptial contract three months before they married in London in 1998 in which Mr Granatino agreed not to claim against her if they separated.
Although the prenuptial contract would have been fully enforceable in Germany or France, last July Ms Radmacher was ordered by a High Court judge in London to pay her estranged husband, Nicolas Granatino, a lump sum of £5,560,000.
Mrs Justice Baron ruled that it would be "manifestly unfair" to hold Mr Granatino to the contract, which was signed in Germany.
Mr Granatino, a French national who worked for JP Morgan, gave up his banking career six years ago to pursue a doctorate in biotechnology at Oxford.
In a statement released after the ruling, Ms Radmacher said: "I am delighted that the court accepts that the agreement Nicolas and I entered into as intelligent adults before our marriage should be honoured.
"Ultimately, this case has been about what I regard as a broken promise.
"When we met and married, Nicolas and I were broadly on an equal footing financially. He too is an heir to a multimillion-pound fortune and, when we met, was an investment banker earning up to £330,000 a year.
"The agreement was at my father's insistence as he wanted to protect my inheritance - this is perfectly normal in our countries of origin, France and Germany. My father taught me the value of hard work and family values.
"Like all wealthy parents, he feared gold-diggers.
"As an heir himself, Nicolas perfectly understood this. The agreement gave me reassurance that Nicolas was marrying me because he loved me as I loved him... that we were marrying for the right reasons.
"The arrangements the court has ordered will enable our daughters to live comfortably when they are with their father, and that is the way it should be. Nicolas and I made each other a promise and all I have been asking is that he be kept to it."
Ms Radmacher 's solicitor Ayesha Vardag said: "For 160 years prenuptial contracts were said to be void for public policy reasons. They were put in the same category as contracts to kill your hated spouse.
"Now, in a landmark judgment, three of the most highly-respected judges in the land have ruled that pre-nups can be decisive in determining the financial division on divorce."
The ruling is likely to be challenged in the House of Lords, although the law lords have said they would wait for the outcome of the Law Commission's review before ruling on the enforceability of pre-nuptial agreements.