A major advertising company has banned Breitbart News from using its ad-serving tools after judging the conservative news site had violated its hate speech rules.

AppNexus analysed Breitbart's website after US President-elect Donald Trump appointed the media company's boss Steve Bannon as his chief strategist.

"We did a human audit of Breitbart and determined there were enough articles and headlines that cross that line, using either coded or overt language," AppNexus spokesman Joshua Zeitz told Bloomberg.

Mr Trump's appointment of Mr Bannon was met with backlash due to Breitbart's appeal to the alt-right movement, which has been accused of racism, anti-semitism and misogyny.

Steve Bannon calls liberal women 'a bunch of dykes'

The President-elect praised Mr Bannon as a "highly qualified leader" and someone who worked well on his presidential campaign.

Mr Bannon declared Breitbart "the platform for the alt-right" after taking over it four years ago.

It is now the most widely-read conservative site in America and was regularly cited by Mr Trump throughout his campaign.

In the run-up to the election, Mr Trump gave several interviews to the site, driving its traffic to a 124 per cent spike in 2016.

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While Breitbart doesn't buy ads directly using AppNexus, it shows ads from the company's customers, according to Bloomberg.

It is unclear what financial impact this may have.

Controversial headlines from the site, including one which praised the confederate flag and another which said "women should log off" when faced with harassment online.