Norwegian Prime Minister Jonas Gahr Støre has said he agrees with Crown Princess Mette-Marit's admission of "poor judgment" after it emerged she had extensive contacts with late sex offender Jeffrey Epstein.

The princess features hundreds of times between 2011-14, in the latest files released by the US Department of Justice relating to Epstein.

This latest embarrassment for the royal family comes on the eve of her son's seven-week trial in Oslo on 38 charges, including rape and assault.

Princess Mette-Marit married Norway's Crown Prince Haakon as a commoner when her son, Marius Borg Høiby, was four, and is in line to become queen upon her husband's accession to the throne.

As the content of her email exchanges began to emerge, the princess released a statement on Saturday saying, "I showed poor judgment and regret having any contact with Epstein at all. It is simply embarrassing."

She went on to express her "deep sympathy and solidarity with the victims of the abuses committed by Jeffrey Epstein".

Although the prime minister did not go further than agreeing with the princess's admission of poor judgment, his implicit criticism of her is considered unusual.

Støre said he also believed that ex-Prime Minister Thorbjørn Jagland had exercised "poor judgement", after it emerged that he had planned a family holiday on Epstein's private island, although he then cancelled it.

The latest cache of Epstein files reveals that Princess Mette-Marit spent four days at Epstein's house in Palm Beach, Miami in January 2013; and in one email from the HRH Crown Princess account, Epstein is asked whether a mother should suggest naked women carrying a surfboard for her son's wallpaper.

The warm nature of the correspondence has appalled commentators in Norway, and one message implied she was aware of his initial jail term in 2008.

Norway's royal family has been buffeted by a succession of scandals in recent times, including the marriage of the crown prince's sister, Princess Märtha Louise, to a self-styled American shaman.

Now as Norwegians prepare for the trial of Marius Borg Høiby, questions are being asked about his mother's failure to realise the toxic nature of maintaining contact with Epstein, and the role of her advisers.

Mette-Marit is suffering from pulmonary fibrosis, and her doctors are preparing to put her on a list for a lung transplant.

Her 29-year-old son, who is not part of the royal family, denies the most serious charges against him. One charge of rape involves intercourse while a woman was asleep. Three others allege sexual assault while women were incapacitated, which also counts as rape in Norway.

If found guilty, he could face at least 10 years in jail.

No member of the royal family will attend the trial, which begins at the Oslo District Court on Tuesday.

However, in an emotional statement to reporters last week, Crown Prince Haakon said "our thoughts are with everyone who is affected by this case...we understand that this is a difficult time for many of you, and we sympathise".

Although his stepson was not a member of the royal family, he said, "We care about him, and he is an important member of our family."