Norway's Crown Princess Mette-Marit has undergone a successful lung transplant in Oslo, the country's royal household has said.

Mette-Marit, 52, was diagnosed with a form of pulmonary fibrosis in 2018. In recent months, her condition had worsened.

"We are delighted that everything has progressed well so far," Are Holm, lung specialist at the hospital, said in the palace statement.

He added that Mette-Marit would remain in hospital for "several weeks to come" under observation, which he said is standard practice for all recent transplant recipients.

Prince Haakon will adjust his official schedule to support her, the palace said.

Høiby, 29, was four when his mother married Crown Prince Haakon and is not himself a royal figure. Ahead of the verdict on Monday, his legal team repeatedly and unsuccessfully sought his release from prison so he could spend time with his mother because of her declining health.

The transplant "is very happy news for the Norwegian royal family and the Kingdom of Norway," said historian and royal commentator Ole-Jørgen Schulsrud-Hansen.

"This was one of the most serious obstacles on the road for a better health for the Crown Princess, and I think many people are relieved the transplant was successful."

Mette-Marit, who married Haakon in 2001, was diagnosed in 2018 with a rare form of pulmonary fibrosis that causes breathing difficulties.

As the illness worsened, she began scaling back her commitments, and earlier this year lung specialist Holm said her deterioration had been significant and "dangerous".

Mette-Marit was last seen in public on 17 May. By that point, she needed a nasal tube connected to an oxygen device.

The palace then announced she had been placed on the transplant list 12 days ago. Her doctors highlighted that the general rule for anyone placed on the list is the patient is believed to have only a year to live.

"We always prioritise the sickest," Holm said.

He also stressed that the recovery period after the transplant would be extremely delicate, with transplant receivers needing to be on immunosuppressive medications for life.

One in eight people who receive a donor lung do not make it through the first year, he said, while about half are alive after ten years.

The lung transplant is the latest challenge in what has been a difficult year for the Norwegian royals. In January, documents were released revealing Mette-Marit's frequent contact with late sex offender Jeffrey Epstein.

She later apologised to Norway's king and queen for her three-year friendship with Epstein and admitted to "poor judgement", adding in a national TV interview that she wished she had never met him.