American news organisation, NowThis, has released footage of Fox News host Tucker Carlson telling Dutch historian Rutger Bregman he has a "tiny brain".
Mr Bregman garnered headlines around the world last month after he accused billionaires at the World Economic Forum in Davos for not paying their fair share of taxes.
On Wednesday, Mr Bregman - through NowThis - released the interview he claims to have been recorded between himself and Mr Carlson.
The interview took place last week and just shows Mr Bregman speaking to the camera, but it was never aired.
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After a few moments, Mr Bregman begins to talk about taxes on the rich and Fox News' role in US politics.
"You're a millionaire funded by billionaires, that's what you are," Mr Bregman told Carlson.
Mr Bregman said many hosts on Fox News have "been against higher taxes" and Mr Carlson was "part of the problem".
"I went to Davos to speak truth to power and I'm doing exactly the same thing right now...you may not like it but you're a millionaire funded by billionaires and that's the reason why you are not talking about these issues."
This prompted Mr Carlson to launch a tirade at the historian.
"Go f**k yourself you [sic] tiny brain and I hope this gets picked up because you are a moron," he said.
"I tried to give you a hearing, but you were too f**cking annoying."
Mr Bregman tweeted the next day asking Fox News to release the full interview.
