Mabel Wisse Smit, 35, had seemed to be the ideal partner for Prince Friso, second son of Queen Beatrix.

A former Balkans expert at the United Nations, and for the past six years a leading human-rights and democracy advocate at the George Soros-funded Open Society Institute, Ms Smit was vetted by the Dutch secret service and approved as a suitable bride.

However, two former associates of Klaas Bruinsma, known as the original Dutch Godfather, alleged that she had once been his girlfriend. Bruinsma, who dominated the Dutch underworld and sold drugs across Europe with 200 criminals working for him, was killed in a gangland gunfight in 1991.

He was notoriously violent. One of his rivals was found upside down in a barrel of cement, having had his penis and his legs cut off while he was still alive.

Charlie Da Silva, the drug baron’s former bodyguard, alleged last week that Ms Smit had a long-term affair with Bruinsma and that she had been aware of his criminal activities.

Mr Da Silva, who now lives in Chile, said that Bruinsma had been smitten by her and that Ms Smit had been the only woman allowed on board his yacht, the Neeltje Jacoba.

Ms Smit has strenuously denied the allegations, saying that she was only an acquaintance of Bruinsma. However, she has admitted that she spent the night on his yacht on several occasions, but denied having had a sexual relationship with him. She insisted that she knew him only for a few months and broke off contact after she found out how he made his money.

A second bodyguard, Guert Roos, has come to Ms Smit’s defence, saying that Mr Da Silva had “a great fantasy” and labelled him a “troll”. Mr Roos said that he could remember Ms Smit only vaguely and he said that countless “girls, women and ladies” passed through Bruinsma’s life and all were excluded from “business” matters.

Bruinsma was born into a wealthy brewing family, but became increasingly involved with drug-dealing. He became one of the biggest drug-dealers across Europe, with heavy involvement in prostitution. However, his total dominance of the Dutch underworld started to unravel after the largest shipment of cannibis resin, 45 tonnes worth £150 million, was busted by police.

He was shot, aged 37, in the early hours after a drinking session at the Amsterdam Hilton ended in an argument.

Prince Friso, a former merchant banker, is due to marry Ms Smit in Delft in April and, if he wants to remain an heir to the throne, he has to have approval for the marriage from parliament.

An opinion poll last week showed that the majority of Dutch people think that the parliament should not approve the marriage and that Ms Smit was lying about her relationship.

Prince Friso has confirmed that he will request permission for the marriage from the Dutch parliament, but the Socialist Party has demanded that the marriage be delayed until the investigation is concluded.

Jan Peter Balkenende, the Dutch Prime Minister, in announcing the inquiry this weekend, said that it would take several weeks to finish. It is expected that the Dutch secret services will be involved and criminologists have said that it must be independent to have any credibility with the Dutch people.

Beatrix Wilhelmina Armgard of Orange-Nassau, now Queen of the Netherlands, first set the trend of upsetting the nation by marrying a German diplomat while memories of the German invasion were still painfully sore.

The eldest son of that union, Crown Prince Willem-Alexander, carried on the tradition by marrying the daughter of a leading member of an Argentinian junta notorious for its human rights abuses. Before Crown Prince Willem- Alexander married Maxima last year, the Government investigated the role that her father, Jorge Zorreguieta, played in the alleged murder of tens of thousands of political opponents by the Videla military regime in Argentina between 1976 and 1983. The investigation cleared Señor Zorreguieta, a senior member of the junta, saying that he probably knew about its human rights violations, but was not personally involved in them.

Señor Zorreguieta himself made a statement to the Dutch people, declaring: “I find each case of abduction, murder or torture unacceptable.

“Until today I am still very much hurt that during the period of office of the government of which I was a part such violations of human rights have taken place.”

Dutch security services also investigated Edwin de Roy Van Zuydewijn, who married Princess Margarita, a niece of Queen Beatrix. He was accused of having controversial business interests, and of unlawfully adopting the title “Baron”.

However, the latest investigation should be the last for a while. The Queen’s third son, the retiring Prince Constantijn, managed to marry two years ago without any scandal.

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