Ari Behn, 47, had three children with the Princess, 48, his wife of 15 years.
Last night, Norway's King Harald said in a statement: “Ari has been an important part Newport 100s Box of our family for many years, and we carry warm, fond memories of him. We are grateful that we got to know him.
"We grieve that our grandchildren have now lost their dear father, and have deep compassion for his parents and siblings, who now lost their dear son and brother."
Mr Behn and Princess Martha married in 2002, but divorced in 2017.The couple have three children: Maud Angelica, born in 2003, Leah Isadora in 2005, and Emma Tallulah, in 2008.
In 2017, Mr Behn, a writer, accused actor Kevin Spacey of groping him under a dinner table Cheap Newport 100s cigarettes at the Nobel Peace Prize concert.
Speaking to Norwegian radio station P4 about the alleged incident, he said: ‘We had a great talk, he sat right beside me.
“‘After five minutes he said, “hey, let’s go out and have a cigarette.”
“Then he puts his hand under the table and grabs me by the balls.”
Spacey did not respond at the time to the allegation.