Hunter Biden is facing federal criminal tax evasion charges, a rarity according to experts. Prosecutors claim that the son of the president evaded $1.4 million in taxes from 2016 to 2019, using the income to support a "lavish lifestyle" involving drugs and escorts.

Despite the charges, Biden, who has achieved sobriety, settled the tax bill in 2020, partly through a loan.

His lawyer has dismissed the allegations as politically motivated. The charges against Biden include nine counts—three felonies and six misdemeanors—such as failure to file and pay taxes, false tax return, and evasion of assessment.

Notably, criminal tax prosecutions, as opposed to civil cases, are deemed very uncommon, with the Internal Revenue Service having significant discretion in deciding when to involve prosecutors, according to Robert Nassau, a tax law expert from Syracuse University.

"If criminal prosecutions weren't rare, there would be lots of people who could be subject to prosecution," Mr Nassau said.

"The criminal statutes cited in this case are pretty broad and could apply to millions of people who don't file a return for one reason or another... prisons could, theoretically, be overfilled with tax criminals."

Civil cases involving much larger sums of money than this one are routine, Mr. Nassau added, calling the $1.4m in the Biden case "small potatoes".

In the charge sheet, prosecutors say Mr Biden repeatedly claimed personal expenses as business deductions while spending millions on "drugs, escorts and girlfriends, luxury hotels and rental properties, exotic cars, clothing, and other items of a personal nature, in short, everything but his taxes".

Much of the evidence now being used against the younger Mr. Biden was laid out in his 2021 memoir, Beautiful Things.

In the book, Mr. Biden details how 2018 - a year in which he claimed $388,810 in business-related travel - was "dominated by crack cocaine use twenty-four hours a day, smoking every fifteen minutes, seven days a week".

The hotels named in the indictment also allegedly correspond with luxury hotels in which he spent much of a months-long drug and alcohol binge with an entourage he recalled would "drink up the entire minibar, call room service for filet mignon and a bottle of Dom Pérignon".

The charges say that he did not, however, "write that he conducted any business in any of these luxury hotels nor did he describe any of the individuals who visited him there as doing so for any business purpose".

Kevin McMunigal, a former federal prosecutor and a professor at Case Western Reserve University, said that the behaviours outlined in the book may have ultimately contributed to the government's decision to charge the case as a criminal matter, rather than a civil one.

"You could imagine a case where maybe somebody was avoiding taxes because they had a family member that was ill, or their home was going to go into foreclosure. Something more appealing than blowing the money on champagne and drugs," he told the BBC.

"I think that would influence the decision about whether or not to go with criminal charges."

Mr McMunigal added that the fact Mr Biden documented his behaviour in his memoir could make the case easier to prosecute.

"Any of those statements are admissible against him," he said.

Mr Biden's attorney has said his client is being prosecuted because of who he is - a Biden.

"Now, after five years of investigating with no new evidence - and two years after Hunter paid his taxes in full - the US Attorney has piled on nine new charges when he had agreed just months ago to resolve this matter with a pair of misdemeanours," Mr Biden's attorney, Abbe Lowell, said in response to the tax charges.

Mr McMunigal said being a Biden "isn't really a defence", but it is an argument that could come into play if he were convicted and his attorneys were trying to reduce his sentence.

He said the tax charges could also serve as leverage for the prosecution.

"I don't quite get why they're adding the tax charges," he said. "The only thing I can think of is maybe they're adding them as a bargaining chip to get him to plead guilty to the gun charges."

The latest tax charges come after a proposed plea deal to solve separate charges that he illegally owned a handgun fell apart earlier this year.

The plea would have seen Mr Biden charged with two misdemeanour counts for failing to pay his taxes on time in 2017 and 2018.

He has since pleaded not guilty on the gun charges.