Viggo Mortensen has rebuked Spain’s far-right party Vox for using an image of his Lord of the Rings character, Aragorn, in a political tweet.

On election day last month, Vox posted an image of Aragorn with the Spanish flag and its party emblem on his back confronting an enemy army under the banners of leftwing, feminist, LGBT and separatist groups. The tweet said: “Let the battle begin! #ForSpain”

In a letter to the Spanish newspaper El País published on Tuesday, Mortensen wrote: “You’d have to be pretty ignorant to think that using the character … to promote the campaign of a xenophobic, ultra-right party like Vox would be a good idea. I would laugh at their ineptitude but Vox has won 24 seats in Congress. It’s no joke.”

Mortensen, a fluent Spanish speaker with a home in Madrid, said Vox’s message was absurd because, in the books by JRR Tolkien, Aragorn was “a multilingual statesman who advocates the knowledge and inclusion of the various races, customs and languages of Middle Earth” and should not be associated with the far right.

Warner Bros Spain, which has local distribution rights to the film, said it had not authorised the use of its copyrighted property in any campaigns.

Mortensen, who is Danish-American, grew up in Argentina and is an outspoken supporter of leftwing causes.