The National Party's copyright case is going to court.

Music label Eight Mile Style is taking legal action over a riff in National's election ad - which it says is a rip-off of Eminem's "Lose Yourself".

The case will be heard in the High Court in Wellington next week.

National changed its ad music during the election campaign, but stood by its use of the tune - which it said it had bought the rights to.

MP and campaign chair Steven Joyce said the party the music was "pretty legal".