US President Donald Trump has reportedly called his condemnation of white supremacists the “biggest” mistake he had ever made.

Axar.az reports that according to an excerpt from Bob Woodward’s new explosive book that was obtained by The Washington Post, Trump told his aides almost immediately after the condemnation last year that it “was the biggest f---ing mistake I’ve made.”

He also called it the “worst speech I’ve ever given,” according to the Pulitzer Prize-winning author's new book, "Fear: Trump in the White House," which is being released on September 11.

Thousands of white supremacists, KKK members and neo-Nazis descended on Charlottesville, Virginia, for a “Unite the Right” rally in August 2017. The march soon turned violent. A 20-year-old man plowed a vehicle into a group of anti-hate demonstrators protesting against the white supremacist rally, killing a woman and injuring 20 others.

Trump first blamed the violence "on many sides", but after pressure against him piled up, he declared that “racism is evil,” singling out white supremacists, neo-Nazis and Ku Klux Klan for the violent rally.

He later said that "we must come together as a nation. I condemn all types of racism and acts of violence."

In the book, Woodward talks about how then-White House staff secretary Rob Porter worked with Trump to write a second speech to try to repair his initial remarks.

Porter tried to persuade the president that he needed to clarify his remarks, however, Trump apparently resisted, repeatedly saying: "I don't know about this. ... This doesn't feel right to me."

After Trump eventually agreed, Woodward writes in the book about how White House aides were relieved, and chief of staff John Kelly encouraged staff to tell the president what he had done was a good thing.