Mary Trump is the daughter of Fred Trump Jr, brother to the current US President. Unlike other members of her family Ms Trump has been sharply critical of the President over a number of issues.
On Thursday her tell-all book ‘Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man’ was published.
The book instantly became a best seller with nearly one million copies sold on the launch day alone.
Speaking to ITV news Ms Trump insisted her presidential uncle is not a reliable ally to the UK.
She said: “For Britain to make certain decisions going forward, they need to know that they can rely on the United States as an ally.
“I don’t believe that you guys can at the moment, in a deep way, as, as you were able to in the past.”
The Trump family tried to block the book’s release but this was rejected by a judge on July 14.
Mary Trump lost her father, President Trump’s older brother, aged just 16 from an alcoholism related condition.
Fred Trump Jr struggled with drink for much of his like and President Trump has previously linked this to his campaign against opioid addiction since taking office.
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She added: “He grew to be an adult who is incapable of looking outside of his own self interests.
“He essentially is stuck on telling himself one story every minute, he’s the best, he’s the greatest.
“He’s always winning because if he loses the thread of that narrative he would have to face things about himself that would be unbearable for him.
“So he has to engage in a lot of projection and other defence mechanisms to protect himself from that, the reality that he’s none of the things he claims to be.
“I believe that the story I tell has a lot of explanatory power and that people will benefit from reading it.”
President Trump has furiously denied the book provides an accurate account of his early life.
The book, published by New York based Simon & Schuster, is currently at the top of Amazon’s bestseller list.
In her book Ms Trump, who holds a doctoral degree in clinical psychology, accused her uncle of being narcissistic and believing that “nothing is ever enough”.
She wrote: “This is far beyond garden-variety narcissism.
“Donald is not simply weak, his ego is a fragile thing that must be bolstered every moment because he knows deep down that he is nothing of what he claims to be.”