American rapper Eminem's court battle with the National Party will take place next year.

The High Court at Wellington has confirmed the trial, which will consider the copyright of music used in the party's 2014 election campaign ads, will start on May 1.

The trial is expected to last three weeks.

Eight Mile Style LLC and Martin Affiliated LLC, Detroit-based publishers of Eminem's copyrights, have accused National of using backing music to the rapper's song Lose Yourself without permission.

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The National Party has flatly rejected the allegation it used the track from the Eight Mile film.

In September 2014, spokesman for the publishers Joel Martin said Eminem was never approached for permission to use his work in National's rowing-themed election ads, which featured backing music similar to the riff of Lose Yourself.

"It is disappointing and sadly ironic that the political party responsible for championing the rights of music publishers in New Zealand by the introduction of the three strikes copyright reforms should itself have so little regard for copyright," Martin said at the time.

The National Party said the music came from an Australian-based production outfit and it had been used by others without complaint.

It's not the first time Eminem has launched legal action over copyright infringement.

In 2004, he sued Apple for using an Eminem song in one of its television commercials without permission.