“We planned a dinner at a restaurant, which we believed to be in line with coronavirus guidelines. It was at a restaurant where we booked three separate tables and where we kept our distance,” Solberg told reporters on Friday.
“I can only apologise for the fact that we did not comprehend that this, as defined by the law, was an event.”
Solberg has championed strict rules to curb the spread of the coronavirus, resulting in some of Europe's lowest rates of infections and deaths, which has boosted support for her Conservative Party in polls ahead of elections in September.
But the country is now seeing a rapid rise in infections led by more contagious variants of the virus, with health officials warning that hospitals risk becoming overwhelmed unless the outbreak is contained.
The birthday party has drawn criticism, including from her own health minister, Bent Hoeie, who told daily VG: “This should not have happened”.