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Gorka, 46, has been accused of ties to far-right groups and his hardline views on immigration and terrorism caused discord within the White House.

A former editor of alt-right Breitbart News website, Gorka joined the administration as a counterterrorism adviser but did not receive top-level security clearance and his exact responsibilities were never clearly outlined.

"Sebastian Gorka did not resign, but I can confirm he no longer works at the White House," an administration official in the White House said after reports that Gorka had left voluntarily.

Gorka, who became a familiar face on television as a staunch advocate of Donald Trump, did not discuss the reasons behind leaving the White House but pointed towards excerpts of his resignation letter posted on Friday evening by The Federalist website

"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," he wrote.

"The best and most effective way I can support you, Mr. President, is from outside the People's House."

In the letter, Gorka also criticised Trump's recent speech on Afghanistan for failing to mention "Radical Islam" or "radical Islamic terrorism".

Born in London to Hungarian parents, Gorka became a US citizen in recent years.

His counter-terrorism knowledge was repeatedly questioned by peers, while it was reported that he is allegedly a "formal member" of a Nazi-allied group Vitezi Rend.

In 2016, Gorka was arrested and charged with possessing a 9mm handgun at the Ronald Reagan Washington National Airport in Arlington, Virginia after the weapon was spotted by an X-ray operator.

Writing at Breitbart, he claimed that President Obama had helped the Islamic State group, compared Obama's administration with the totalitarian regime depicted in the George Orwell novel 1984, and said that the Muslim Brotherhood had over-run the National Cathedral in Washington DC.

Gorka's departure is the latest high-profile exit from the White House after far-right chief strategist Steve Bannon, who left the Trump administration last week.

Agencies contributed to this report.