So she appeared to be the perfect bride for Lord Nicholas Windsor when the Queen gave her blessing to their union — expected to be the Royal Family’s first wedding at the Vatican for 400 years. But inquiries by The Times have uncovered the extraordinary story of how the Princess’s family has reinvented itself over the past decade.

Buckingham Palace may have been unaware of the transformation when it announced the couple’s engagement this week. A Court Circular in The Times noted: “The Duke and Duchess of Kent are pleased to announce the engagement of their son Nicholas to Paola, daughter of Don Louis and Donna Ingrid Doimi de Frankopan of London.”

Princess Paola, 37, and Lord Nicholas, 36, met at a party in New York five years ago. He had recently converted to Roman Catholicism, forfeiting his place as 26th in line to the throne.

The couple’s shared faith led them to plan their wedding at St Peter’s Basilica in November. The Prince of Wales, the groom’s godfather, is expected to be among the aristocratic guests from all over Europe.

Before following his mother into the Catholic Church, Lord Nicholas — the Queen’s first cousin once removed — had had a troubled past. A police caution for cannabis possession when he was 18 was followed by reports of an eating disorder and difficulty finding a career.

His bride has also undergone change. Her father’s family fled to England from what is now Croatia during the Second World War. Louis Doimi di Delupis, as he was then known, married a Swedish lawyer, Ingrid Detter, and became a successful businessman. It was only after the fall of the Iron Curtain that the family began to refer to themselves as the Frankopans.

Frankopan is Croatia’s most aristocratic title and, in a country that only recently regained its independence, is a highly evocative name. Fran Krsto Frankopan led its campaign for freedom from Austria in the 17th century until he was executed by Leopold I in 1671.

The family’s assets are said to include investment companies and a Zagreb office block. Last month they announced plans to invest up to £68 million in property and equities. But documents at Companies House, in London, show that they legally adopted the title Frankopan in February 2000. Before then they were simply called Doimi de Delupis, the name on their birth certificates.

It emerged yesterday that Prince Louis had been denied membership of the Croatian Nobility Association because of the disputed use of his title. Ivo Durbesic, the association’s president, told The Times: “He took the name Frankopan, but he is not a descendant of the Frankopan family. He is aristocratic but derives from the Doimi de Lupis bloodline. We strongly disliked the fact that he was using the name Frankopan without any proof that he was related to that family.”

The Encyclopaedia Yugoslavika records that, after Prince Fran Krsto was executed, the only Frankopan to survive was Urio, whose line died out in the late 19th century.

Nepo Kuzmanic, a leading Croatian genealogist, said: “There is no family tree or genealogy available to prove the noble continuity.”

John Kennedy, editor of the Almanach de Gotha, the directory of Europe’s nobility, said: “You can call yourself whatever you want, but it is more an aspirational title than one they have inherited.”

He said that the failure of the Court Circular to refer to the Frankopans as Prince and Princess showed that Buckingham Palace did not accept the use. “Don and Donna” simply acknowledges their claim to be members of the nobility.

However, Paola’s brother, Prince Peter, 35, defended the use of the name and titles. “The title is not any claim on anything. It is just a reflection of the age of the family,” he said.

He said that his branch of the family had split from the Frankopans in the 13th century. “In 1991 we took back the name of Frankopan. It was considered by my family and the government at the time as an appropriate thing to do.”

THE GREAT PRETENDERS

• Anthony Hall campaigned to prove he was the rightful heir to the throne as a descendant in a direct male line from Henry VIII. His public tour in the 1930s so alarmed the monarchy that George V agreed that Hall, a former police inspector be prosecuted for “scandalous language”
• The disappearance of Grand Duchess Anastasia Romanov, the youngest daughter of Nicholas II of Russia, during the Bolshevik revolution led to many people claiming her identity. Anna Anderson, the most famous claimant, was proved to be a charlatan after DNA tests in 1994
• Princess Caraboo became a darling of 19th-century high society with her striking, exotic looks and previously unknown language. But she was in fact a cobbler’s daughter and servant girl from Devon called Mary Baker, who was born in 1791 and who reinvented herself after turning up at hospital for prostitutes in London

Times Recommends

• 'Great Schlep' may seal deal for Obama
• Hindu 'storm troopers' flex their muscles
• Ahtisaari: deserved reward for (failed) plan