President Joe Biden’s son Hunter’s plea deal on two tax charges has fallen apart after the federal judge hearing his case expressed concern over a related agreement on a more serious gun possession charge.

The outcome leaves open, at least temporarily, the years-long investigation into Hunter Biden’s business dealings.

He had been charged with two misdemeanour tax crimes of failure to pay more than 100,000 dollars (£70,000) in taxes from over 1.5 million dollars (£1.16m) in income in both 2017 and 2018, and he made an agreement with prosecutors, who were planning to recommend two years of probation.

Last month, Biden was charged with possession of a firearm by a person who is a known drug user. He had a Colt Cobra .38 Special for 11 days in October 2018.

Hunter Biden
Under the plea deal, Hunter Biden would have been spared time behind bars (Andrew Harnik/AP)

He agreed to enter into a diversion agreement, which means that he would not technically plead guilty to the crime.

As long as he adhered to the terms of his agreement, the case would be wiped from his record. If not, the deal would be withdrawn. The charge carries a maximum sentence of 10 years in prison.

US District Court Judge Maryellen Noreika, who was appointed by President Donald Trump, said she was concerned about the language in the diversion agreement and suggested the lawyers get back together and discuss it.

“I think having you guys talk more makes sense,” she said.

Republicans had argued Biden was getting a sweetheart deal.

It was not immediately clear whether the lawyers could resolve the case on Wednesday or whether they would have to go back into more discussions.

The deal was meant to clear the air for Hunter Biden and avert a trial that would have generated weeks or months of distracting headlines.

But the politics remain as messy as ever, with Republicans insisting he got a sweetheart deal and the Justice Department pressing ahead on investigations into Mr Trump, the Republican 2024 presidential primary front-runner.

Mr Trump is already facing a state criminal case in New York and a federal indictment in Florida.

But last week, a target letter was sent to Mr Trump from special counsel Jack Smith that suggests the former president may soon be indicted on new federal charges, this time involving his struggle to cling to power after his 2020 election loss to Joe Biden.

Republicans claim a double standard, in which the president’s son got off easy while the president’s rival has been unfairly castigated. Congressional Republicans are pursuing their own investigations into nearly every facet of Hunter Biden’s dealings, including foreign payments.

On Tuesday, a row arose after Republicans on the House Ways and Means Committee filed court documents urging Judge Noreika to consider testimony from Internal Revenue Services (IRS) whistleblowers who alleged Justice Department interference in the investigation.