The former White House adviser Steve Bannon has described the Trump Organization as a criminal entity and predicted that investigations into the president’s finances will lead to his political downfall, when he is revealed to be “not the billionaire he said he was, just another scumbag”.

The startling remarks are contained in Siege: Trump Under Fire, the author Michael Wolff’s forthcoming account of the second year of the Trump administration. The book, published on 4 June, is a sequel to Fire and Fury: Trump in the White House, which was a bestseller in 2018. The Guardian obtained a copy.

In a key passage, Bannon is reported as saying he believes investigations of Donald Trump’s financial history will provide proof of the underlying criminality of his eponymous company.

Assessing the president’s exposure to various investigations, many seeded by the special counsel Robert Mueller during his investigation of Russian election interference, Wolff writes: “Trump was vulnerable because for 40 years he had run what increasingly seemed to resemble a semi-criminal enterprise.”

He then quotes Bannon as saying: “I think we can drop the ‘semi’ part.”

Bannon, a leading promoter of far-right populism, was a White House adviser until August 2017, when he was removed. He was a major source for Fire and Fury, also first reported by the Guardian. Among other claims in that book, he labelled as “treasonous” an infamous Trump Tower meeting between Donald Trump Jr, Trump’s son-in-law Jared Kushner, campaign manager Paul Manafort and a Russian lawyer.

Amid publicity surrounding Fire and Fury, Bannon was ejected from circles close to Trump and his position at Breitbart News.

In Siege, Wolff pays close attention to Trump’s financial affairs. Investigations into Trump’s business dealings, spearheaded by the southern district of New York, have shuttered the president’s charity and seen the Trump Organization chief financial officer, Allen Weisselberg, receive immunity for testimony in investigations of Michael Cohen, the former Trump attorney and fixer who is now in jail in New York.

This month, the New York Times obtained tax information that showed Trump’s businesses lost more than $1bn from 1985 to 1994.

The newspaper subsequently reported that in 2016 and 2017, Deutsche Bank employees flagged concerns over possible money laundering through transactions involving legal entities controlled by the president and Kushner. Some of the transactions involved individuals in Russia.

The bank did not act but Congress and New York state are now investigating its relationship with Trump and his family. Deutsche Bank has lent billions to Trump and Kushner companies. Trump has attempted to block House subpoenas for his financial records sent to Deutsche Bank.

In Siege, Wolff quotes Bannon saying investigations into Trump’s finances will cut adrift even his most ardent supporters: “This is where it isn’t a witch hunt – even for the hard core, this is where he turns into just a crooked business guy, and one worth $50m instead of $10bn.

“Not the billionaire he said he was, just another scumbag.”

Wolff also details a 2004 Palm Beach property deal involving the now disgraced financier Jeffrey Epstein and the Putin-friendly oligarch Dmitry Rybolovlev that, the author writes, earned Trump “$55m without putting up a dime”.

Epstein, he writes, invited Trump to see a $36m Palm Beach mansion he planned to buy. According to Wolff, Trump went behind Epstein’s back to buy the foreclosed property for around $40m, a sum Epstein had reason to believe Trump couldn’t raise in his own right, through an entity called Trump Properties LLC, which was entirely financed by Deutsche Bank.

Epstein, Wolff writes, knew Trump had been loaning out his name in real estate deals for a fee and suspected that in his case Trump was fronting for the property’s real owners. Epstein threatened to expose the deal. As the dispute increased, he found himself under investigation by the Palm Beach police.

According to Wolff, Trump made only minor improvements and put the house on the market for $125m. It was purchased for $96m by Rybolovlev, part of a circle of government-aligned industrialists in Russia, thereby earning Trump $55m without risking any of his own money.

Wolff presents two theories as to how the deal worked: first, perhaps “Trump merely earned a fee for hiding the real owner – a shadow owner quite possibly being funneled cash by Rybolovlev for other reasons beyond the value of the house”.

Second, he suggests the real owner of the house and the real buyer were one and the same. “Rybolovlev might have, in effect, paid himself for the house, thereby cleansing the additional $55m for the second purchase of the house.”