NEW YORK • US President Donald Trump's niece describes him as a lying narcissist who was shaped by his domineering father, according to excerpts of her eagerly anticipated memoir carried in United States media on Tuesday.

Ms Mary Trump's Too Much And Never Enough: How My Family Created The World's Most Dangerous Man is due out next Tuesday amid a legal battle to stop its publication, and is already a bestseller on Amazon.

In it, she accuses Mr Trump of "hubris and wilful ignorance" stretching back to his younger days, according to CNN, which has seen a copy.

She writes that Mr Trump developed "twisted behaviours" and saw "cheating as a way of life," according to the New York Times (NYT).

Ms Trump alleges that the future US leader cheated on an exam, helping him get into the University of Pennsylvania's prestigious Wharton Business School, the newspaper reported.

She says her uncle paid someone else to take the pre-collegiate test when he was a high school student in New York. NYT did not say how she knew this.

The 240-page book says Mr Trump is a product of his "sociopath" father Fred Trump Sr, The Washington Post (WP) reported.

Ms Trump, 55, a clinical psychologist, says her uncle failed to develop human emotion because his father created an abusive and traumatic home life.

She says that for the future US leader, "lying was primarily a mode of self-aggrandisement meant to convince other people he was better than he actually was," WP said.

While several close associates of Mr Trump have published exposes of him and his time in office, including former aide John Bolton's tome, which describes Mr Trump as corrupt and incompetent, Ms Trump, who lives in Long Island in New York, is the first member of the family to have broken ranks by writing a book.

The memoir is billed as the first unflattering portrayal of Mr Trump by a family insider.

The President's younger brother Robert Trump went to court to try to block its publication, arguing that Ms Trump was violating a non-disclosure agreement signed in 2001 after the settlement over her grandfather's estate.

Last week, a New York appeals judge ruled that Simon & Schuster is allowed to release the memoir, saying it was "not a party to the agreement".

On Tuesday, White House spokesman Kayleigh McEnany described it as "a book of falsehoods", even though she admitted she had not seen it.

Ms McEnany told reporters: "It's ridiculous, absurd allegations that have absolute no bearing in truth."

Ms Trump is the daughter of Mr Fred Trump Jr, Mr Trump's older brother, who died in 1981 from complications related to alcoholism.

Mr Trump Sr died in 1999.

She and her brother, Mr Fred Trump III, were the only children of Mr Trump Jr and Mrs Linda Clapp Trump, a one-time flight attendant who did not win her father-in-law's approval.

Mr Trump Jr was not inclined to take over the family's real estate business, so his younger brother, Mr Donald Trump, stepped into the role of his father's successor.

The older Trump brother became a pilot and struggled with alcoholism.

In her book, Ms Trump writes that her uncle watched her grandfather mock her father, learnt from the ridicule to become the favourite son, and joined in it.

Mr Donald Trump told his brother, referring to his career as a pilot: "Dad's right about you: You're nothing but a glorified bus driver."

Her father started to spiral downwards.

Mr Trump Jr died of a heart attack in 1981 at the age of 42.

Ms Trump writes that her uncle meets all the clinical criteria for being a narcissist, according to NYT.

"Donald's pathologies are so complex and his behaviours so often inexplicable that coming up with an accurate and comprehensive diagnosis would require a full battery of psychological and neurophysical tests that he'll never sit for," she writes.

The memoir is also due to reveal that Ms Trump was the crucial source for explosive NYT reporting on Mr Trump's finances, which suggested the billionaire paid little in tax for decades, according to The Daily Beast.

When her uncle announced that he was running for president in June 2015, Ms Trump did not take it seriously, assuming, she writes, that he "simply wanted the free publicity for his brand".

Throughout the campaign, which was marked by scandals like the release of the "Access Hollywood" tape, Ms Trump did not speak out, fearing that her voice would not be heard and that her views would make no difference, she writes in the book.

She stayed in touch with her aunt Maryanne Trump Barry, President Trump's older sister, whom she quotes as saying about the presidential race, "He's a clown - this will never happen", during one of their regular lunches in 2015.

Ms Barry was particularly baffled by support for her brother among evangelical Christians, according to the book.

AGENCE FRANCE-PRESSE, NYTIMES