Norway's government announced Saturday that defence minister Odd Roger Enoksen was resigning following revelations that he had a years-long affair with a much younger woman.

"It is a necessary decision, after what has surfaced in this matter," Prime Minister Jonas Gahr Store told a press conference where he confirmed that he had accepted Enoksen's resignation.

"I have made several bad choices and judgements, and will give an unreserved apology for the fact that my actions have made life more difficult for others," Enoksen told news agency NTB earlier on Saturday.

Enoksen started the affair in 2005 when his partner was still an 18-year-old high school student, according to daily VG.

Her class went on a school trip to Norway's parliament in the capital Oslo and met the Centre Party politician, who was 50 at the time.

After the school trip she and Enoksen started what would become a very close and sexual relationship, VG wrote.

The woman, now in her 30s, detailed their relationship to the newspaper, saying she felt starstruck by Enoksen due to her interest in politics and that he "used his power and position to get what he wanted".

According to the woman, they met at least 12 times in Enoksen's office between late 2006 and 2007 while he was the energy minister, with some of the meetings involving "sexual acts".

While Enoksen confirmed to VG that she had visited him in his office and said there might have been some talk of a lewd nature, he denied there was any physical aspect.

He instead claimed they did not become intimate until after he left the government in 2007, and stressed that the relationship was not one "where I was in a position of power over her". 

Prime Minister Store told reporters that Enoksen had not informed him about the affair before he was appointed defence minister following last year's general election, adding the information would have led to a "different conclusion".

Store added that he wished to find a new defence minister as soon as possible.