Hunter Biden has pleaded not guilty to tax and gun charges amid ...

US President Joe Biden's son Hunter Biden has pleaded not guilty to federal tax and gun charges, after a plea deal that was intended to resolve the allegations fell apart in court. 

The stunning turnabout came after what was expected to be a routine plea hearing turned into a three-hour affair featuring hushed negotiations between lawyers and pointed questions from US District Court Judge Maryellen Noreika.

Hunter Biden, 53, has originally pleaded guilty to tax-related charges in an initial appearance in a federal court in Delaware and reached a plea deal with federal prosecutors that averted a conviction on a gun-related charge as he pleaded guilty to two misdemeanour charges of wilfully failing to pay income taxes. 

The judge said she could not accept Hunter Biden's plea deal. (Reuters: Jonathan Ernst)

However, the agreement fell apart when the judge, who was appointed by former president Donald Trump, raised multiple concerns about the specifics of the deal and her role in the proceedings.

The plea plan included an agreement on a separate gun charge; Hunter Biden has been accused of possessing a firearm in 2018 as a drug user.

As long as he adhered to the terms of his agreement, the gun case was to be be wiped from his record. Otherwise, the felony charges carry 10 years in prison.

The overlapping agreements created confusion for the judge, who said the lawyers needed to untangle technical issues — including those relating to her role in enforcing the plea deal — before moving forward.

"I cannot accept the plea agreement today," she said.

She said she did not want to "rubber stamp" a plea deal and needed more time to review it. 

The collapsed proceedings were a surprising development because the plea had been carefully negotiated over weeks and included a lengthy back-and-forth between Justice Department prosecutors and Mr Biden's attorneys.

Here's how a "period of turmoil" landed him here. 

Court documents allege that he failed to pay more than $US200,000 ($295,000) in federal income taxes for 2017 and 2018.

The federal probe began in 2018, before he revealed that he had received a subpoena as part of the Justice Department's scrutiny of his taxes.

The subpoena sought information on Hunter's business dealings with a number of entities, including Burisma, a Ukraine gas company whose board he joined in 2014.

It sparked concerns about the perceptions of a conflict of interest, given Joe Biden was deeply involved in US policy toward Ukraine.

Hunter Biden pictured with his father Joe Biden in 2010. (Reuters: Jonathan Ernst)

An investigation by the then-Republican-controlled Senate did not identify any policies that were directly affected by Hunter’s work.

The younger Biden has also reached an agreement with the Justice Department on a charge that he illegally possessed a firearm while being a drug user.

Federal law prohibits people who use drugs from possessing firearms or ammunition, although a federal judge challenged the legality earlier this year.

According to the Justice Department's Bureau of Alcohol, Tobacco, Firearms and Explosives, between 1998 and 2014, nearly 100,000 prospective gun purchasers went home empty-handed because they were flagged as using illegal drugs.

A lawyer for Hunter said in a statement that he made "these mistakes ... during a period of turmoil and addiction in his life" and he believed it was important to take responsibility. 

Hunter detailed his "deep descent" into substance addiction, after the 2015 death of his brother Beau, in a memoir released in 2021.

He described a lifelong struggle with alcohol and drug abuse, from getting drunk on beers at 14, to bailing on rehab to smoke "crack I'd tucked away in my travelling bag" before he got clean shortly later. 

"In the last five years alone, my two-decades-long marriage has dissolved, guns have been put in my face, and at one point I dropped clean off the grid, living in $59-a-night Super 8 motels off I-95 while scaring my family even more than myself," he wrote in the book. 

A recent poll found most respondents wouldn't let the case affect their likelihood of voting for the elder Biden next year when he is seeking re-election.

The Reuters/Ipsos poll did however find that half of Americans believed the president's son received preferential treatment from prosecutors.

The two-day poll in June showed Americans were divided along partisan lines in their views on the case, with 75 per cent of Republicans seeing preferential treatment compared with just 33 per cent of Democrats.

But most said it wouldn't sway the way they vote. 

Some Republicans have aired their beliefs of preferential treatment, and are questioning why Delaware US Attorney David Weiss, the prosecutor overseeing the  case, didn't bring more aggressive felony charges.

Their criticism has been fuelled in part by claims from Gary Shapley, an IRS criminal supervisory agent who worked on the Hunter Biden investigation.

In an interview with politicians, he claimed that the Justice Department repeatedly stonewalled the probe, and that investigators uncovered evidence of more serious tax crimes that could only be pursued in either Washington, DC or California, but not in Delaware.

Mr Shapley claimed that Mr Weiss sought permission from Attorney General Merrick Garland to be designated as special counsel — a status that would have allowed him to bring federal charges in any district across the nation — and that the request was denied. 

Mr Weiss in a letter to Republican Senator Lindsey Graham said he has "never been denied the authority to bring charges in any jurisdiction". 

Mr Garland also denied the claim, saying Mr Weiss was given "complete authority".

Hunter Biden's attorney denied that his client received any special treatment.

AP/Reuters