"Sebastian Gorka did not resign, but I can confirm he no longer works at the White House," a White House official said.
Gorka was a close associate of former White House chief strategist Steve Bannon, who was ousted last week.
His removal was one of the key demands of Democratic lawmakers led by Indian-American Pramila Jayapal, who has said Gorka represented the dangerous rightwing in the White House.
"Given recent events, it is clear to me that forces that do not support the MAGA (Make America Great Again) promise are - for now - ascendant within the White House," Gorka wrote in his resignation letter, first reported by Federalist.
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"As a result, the best and most effective way I can support you, Mr President, is from outside the People's House," he wrote.
According to Federalist, Gorka's letter expressed unhappiness with the direction the Trump administration's foreign policy has taken, as signalled by the president's recent speech on Afghanistan: "Regrettably, outside of yourself, the individuals who most embodied and represented the policies that will 'Make America Great Again,' have been internally countered, systematically removed, or undermined in recent months. This was made patently obvious as I read the text of your speech on Afghanistan this week..." he wrote.
"This omission should seriously disturb any national security professional, and any American who is unsatisfied with the last 16 years of disastrous policy decisions which have led to thousands of Americans killed and trillions of taxpayer dollars spent in ways that have not brought security or victory," Gorka said.
In recent weeks reports had emerged that he had ties with Nazism, an allegation that was strongly refuted by him.
"Millions of people believe in, and have chosen, you and your vision of Making America Great Again. They will help eventually rebalance this temporary reality," he said.
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