The founder of the Ikea furniture chain, Ingvar Kamprad, is facing further questions about his Nazi past following claims in a new book.
Author Elisabeth Asbrink says Mr Kamprad was an active recruiter for a Swedish Nazi group, and stayed close to sympathisers well after World War II.
The details go beyond what Mr Kamprad has previously admitted.
The Swedish billionaire has said his involvement was youthful "stupidity", and the "greatest mistake" of his life.
In her book, Ms Asbrink says Mr Kamprad actively recruited people to the fascist Sweden's Socialist Union (SSS).
She says the activity prompted security police to set up a file on him in 1943 when he was 17 - the same year that he founded Ikea.
Ms Asbrink says the security police intercepted his post, and noted that he "had some sort of functionary position" in a youth Nazi organization.
The intelligence services have refused to comment.
'Proud of links'
The tycoon revealed some elements of his past in a book in 1988, admitting that he was a close friend of the Swedish fascist activist Per Engdahl, and a member of his New Swedish Movement between 1942 and 1945.
Ms Asbrink's book includes details of a wedding invitation Mr Kamprad sent to Engdahl in 1950, telling him how proud he was that the two belonged to the same circle.
She says that, in an interview in 2010, he told her that: "Per Engdahl is a great man, and I will maintain that as long as I live."
A Swedish expert on far-right extremism, Anna-Lena Lodenius, told Radio Sweden that Mr Kamprad's Nazi involvement could no longer be dismissed as the by-product of an accidental friendship with Per Engdahl.
His involvement in another fascist organisation, she said, showed he must have been "perfectly aware" of what it stood for.
However, a spokesman for Mr Kamprad said he had long admitted flirting with fascism, but that now, "there are no Nazi-sympathising thoughts in Ingvar's head whatsoever".
Although Mr Kamprad ranks 162nd in the most recent Forbes magazine wealth list, worth an estimated $6bn (£3.6bn), some analysts believe he is much richer.
Ownership of Ikea, the world's largest furniture chain, is now in the hands of a Dutch charitable foundation created by Mr Kamprad. But some, including the Swedish business newspaper Veckans Affarer, believe that behind complicated legal arrangements, it is still effectively his company.
Even so, Mr Kamprad is renowned for a devotion to frugality, reportedly driving an old Volvo and travelling by economy class.