The former Breitbart editor at the centre of email exchanges showing senior White House adviser Stephen Miller promoted racist conspiracy theories has said she believes he is a white supremacist.

Katie McHugh swapped hundreds of messages with Donald Trump’s now-senior policy wonk while she worked at the right-wing website between 2015 and 2016.

The emails show him appearing to advocate the so-called Great Replacement theory, a cornerstone of extremist right-wing thinking which suggests immigration is a front for the long-term demographic ousting of white Americans and Europeans.

They were published last month by the Southern Poverty Law Center, a campaigning legal organisation in the US.

Now, Ms McHugh – who admits she too was a racist – has raised concerns that a man with such a world view is directly influencing the country's foreign and domestic policy.

"I would absolutely call him a white supremacist," the 27-year-old told CNN, adding: "I believe his ideology is one of domination and control of people of colour."

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She said: “I just think it's important to speak about this publicly because people need to know. It's a serious danger and I see other people younger than me going down that same path."

And she added: "I do want to emphasise ... that those emails are now White House policy."

Perhaps the most obvious example comes from an exchange in October 2015 in which Ms McHugh asks Mr Miller if he thought a natural disaster in Mexico could drive people to the US border.

"100 percent," he replies before raising the possibility that those potential migrants could be allowed to stay in the US with Temporary Protected Status, an exemption-of-leave given to citizens who have faced environmental disaster, war or extraordinary conditions in their own country.

The Trump administration has since decided not to offer this status to survivors of Hurricane Dorian which destroyed vast swathes of the Bahamas this summer and is in the process of rescinding it for people from El Salvador, Haiti, Honduras, Nepal, Nicaragua and Sudan. The government has said the original dire conditions in those countries are no longer present.

Mr Miller is considered instrumental in having shaped those policies.

Ms McHugh says she was introduced to Mr Miller in June 2015 when he was working for then-Senator Jeff Sessions

“Miller was introduced to me as someone that I would take editorial direction from as I was reporting on the immigration beat and criminal justice beat," she said. "It was not like, 'Here's someone from a Senate office, he may pitch you stories.' It was understood that Miller had editorial control over the political section.”

Of her own extremist views, she added: “I was a white nationalist. Whatever you want to call it -- white nationalist, white supremacist. But that part is dead…I was able to break away from what was frankly a toxic culture and a radicalisation machine, especially for young people like me."

Breitbart has since said she was sacked.

Elizabeth Moore, vice president of public relations and communications for Breitbart, said in a statement: "This person (McHugh) was fired years ago for a multitude of reasons, including lying.”

The White House has not commented on the new interview but has defended Mr Miller as someone who "condemns racism and bigotry in all forms".