US President Joe Biden's son Hunter will be charged before the end of September with a federal firearm offence after his plea deal went sour, the prosecutor has said in a court filing.

Special Counsel David Weiss, who has investigated the president's son for five years, said on Wednesday a grand jury would issue its indictment in the case before September 29.

"The government intends to seek the return of an indictment in this case before that date," Weiss wrote in a status report to US District Judge Maryellen Noreika.

According to the original Weiss charges filed in June, Biden failed to file his tax returns on time on earnings of more than $1.5 million for 2017 and 2018.

He also bought and kept a handgun for a few weeks in 2018 even though he had, as he has openly admitted, a serious drug problem.

In a deal reached with Weiss in late July, Biden agreed to plea guilty to two minor tax charges.

In exchange, he was offered probation, as he had already paid what he owed the government, along with penalties.

In the same deal, Weiss agreed to suspend the felony gun charge if Biden completed "pretrial diversion," which often involves counselling or rehabilitation.

But in a dramatic July 26 hearing, the deal collapsed over the issue of whether Biden would have been immune from any other charges also investigated by Weiss, including possible crimes related to his business dealings in Ukraine, China and elsewhere.

The judge mentioned the possibility that Biden could be charged as having acted as a lobbyist for foreign governments without registering with the Justice Department.

Three weeks later, after the deal collapsed, Weiss dropped the tax charges and indicated in a court filing that new charges would be brought in other states.

And on Wednesday, he told the Delaware court that Biden would be charged in the gun case, which could bring up to 10 years in prison.

'Sweetheart deal'

The legal troubles of Biden, 53, a Yale-trained lawyer and lobbyist, have cast a shadow over his father's reelection campaign.

The case has become a political lightning rod.

Republicans in Congress have raised the possibility of impeaching President Biden over his son's business dealings and accuse the Department of Justice of giving him a "sweetheart deal."

US Attorney General Merrick Garland has denied Republican accusations of favouritism and elevated Weiss from US Attorney for the District of Delaware, who has been investigating the case since 2019.

As special counsel, Weiss, an appointee of former president Donald Trump, has additional authority.

If the case goes to trial, the president would be campaigning for reelection in November 2024, likely against former president Donald Trump, as his son faces criminal prosecution.