Mr Schwartz has been a fierce critic of the billionaire property magnate since he threw his hat into the ring for the presidential race: AFP/Getty Images

The co-author of Donald Trump’s memoir The Art of the Deal has predicted the US President is going to resign by autumn if not sooner.

Tony Schwartz, who claims to have ghostwritten the 1987 best-selling business book, argued Mr Trump is on the brink of stepping down and said he would be shocked if his presidency lasts until the end of the year.

Mr Schwartz, who has been a vocal critic of President Trump and spent 18 months interviewing and shadowing him in the 1980's, suggested he would negotiate a deal for immunity in the Russia investigation in exchange for giving up his seat in the Oval Office.

He said: “The circle is closing at blinding speed. Trump is going to resign and declare victory before Mueller and Congress leave him no choice”.

“Trump's presidency is effectively over. Would be amazed if he survives till the end of the year. More likely resigns by fall, if not sooner.”

he has gone over the line of acceptability so far there is no return. the Russia stuff will be huge. he doesn't want to go to jail. — Tony Schwartz (@tonyschwartz) August 17, 2017

The author argued resistance against the president, who has drawn fierce criticism for drawing moral equivalence between white supremacists and anti-fascists in the wake of Charlottesville, must be maintained despite his resignation being imminent.

He said: “Trump must be isolated. Resistance every day. The end is near but must keep pressure high.”

He added: “The end game is on: Trump goes down or we do. He will blow up the world to prove he matters. We must stand up in opposition every day".

Mr Schwartz has been a fierce critic of the billionaire property magnate since he threw his hat into the ring for the presidential race and has often raised alarm bells about the danger of him having access to America’s nuclear weapons.

Last year he announced he was advising the campaign of Mr Trump’s Democrat rival Hillary Clinton for free as “penance” for boosting Mr Trump’s success and profile by ghostwriting his book.

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“This is my penance for having created a man who has become a monster, and I’ve spent 30 years feeling bad about it,” he told CNN ahead of the second Presidential debate. “Now I feel like I’ve got to show there’s nobody behind the curtain.”

When pressed about whether there was anything he found appealing about Mr Trump, he said “nothing”.

“This is a man who I really believe lacks a conscience at the deepest level, so there really was nothing. He was effective in certain ways, he’s a dominant, aggressive personality and he pushes and he pushes and he pushes and he gets a lot of what he wants.”

Mr Schwartz claims to have gotten to know Mr Trump very well while working on The Art of the Deal, saying he spent hundreds of hours talking to him about his life, listening to him speak and generally observing him in action.

He has since said he bitterly regrets the way he portrayed the former reality TV star, saying the glowing depiction is a piece of fiction and amounted to himself having “put lipstick on a pig”.

On top of this, he said if he could go back in time and rewrite it he would opt for the title The Sociopath. Last autumn, he even committed to giving $55,000 in royalties he had made off the book over the past six months to the National Immigration Law Center. This is an American organisation which focuses on achieving social, economic, and racial justice for low-income immigrants and pushes for policies that would allow more undocumented immigrants to remain in the US legally.

There have been some disputes between Mr Schwartz and Mr Trump about who wrote the book. The president has challenged Mr Schwartz' position as the ghostwriter via his legal team but Mr Schwartz claims he has proof from his publisher Random House that he wrote the entire book.

Mr Schwartz, who shared credit as a co-author with Mr Trump, claims Mr Trump only did some minor editing of the book which he touted during his campaign as evidence of his skills as a negotiator.

Mr Trump has acknowledged Mr Schwartz as his co-author but insisted he wrote his own memoir, saying: “He owes a lot to me. I helped him when he didn’t have two cents in his pocket. It’s great disloyalty. I guess he thinks it’s good for him - but he’ll find out it’s not good for him.”