The wife of one of Norway’s richest men has been abducted from her home outside Oslo, with the kidnappers demanding a ransom of €9 million in cryptocurrency.

Tom Hagen returned home on October 31 to find his wife Anne-Elisabeth Falkevik Hagen missing and a ransom note, written in bad Norwegian, threatening to kill her unless he made a payment of €9 million in the crypto currency Monero.

“There have been both ransom demands and serious threats,” Tommy Brøske, the police inspector leading the investigation, said at a press conference in Oslo on Wednesday morning. “Police have so far advised the family not to meet the requirements.”

Mr Hagen, 68, has made an estimated 1.7bn Norwegian kronor (£156m) from Elkraft, the power supply company he founded, ranking him 172nd on the Norway rich list by Kapital, a financial magazine. It is the country's first kidnap for crypto ransom.

Norwegian police had asked the media to stay silent on the kidnap, but on Wednesday decided to make it public in the hope that neighbours, or people hiking in the area, may have seen or recorded something that could help lead to the attackers.