A US tax special agent has information about how a criminal investigation into the son of US President Joe Biden has been hampered by "political considerations", says his lawyer.

The attorney for the unnamed agent says his client is seeking whistleblower protections from Congress to speak out.

He says the agent alleges "preferential treatment and politics improperly infecting decisions" in the inquiry.

A White House official denied Mr Biden had meddled in the probe into his son.

Congressional Republicans are investigating whether Hunter Biden improperly used family ties in his business dealings.

Mark Lytle, a lawyer representing the employee of the Internal Revenue Service (IRS) tax agency, wrote to a bipartisan group of lawmakers on Wednesday.

He said he represents a "career IRS Criminal Supervisory Special Agent who has been overseeing the ongoing and sensitive investigation of a high profile, controversial subject since early 2020". 

The employee is prepared to "contradict sworn testimony to Congress by a senior political appointee", and reveal "clear conflicts of interest", the letter states.

"My client wants to come forward to Congress," Mr Lytle told CBS News.

"He's ready to be questioned about what he knows and what he experienced under the proper legal protections."

The lawyer added that his client was limited from speaking publicly about any individual's tax filings, and would require protections from Congress in order to reveal any insights.

A lawyer for Mr Biden told NBC News that the IRS agent appeared to have "committed a crime".

"It is a felony for an IRS agent to improperly disclose information about an ongoing tax investigation," said the attorney, Chris Clark.

In October, a US source told reporters that the FBI had collected enough evidence to charge Hunter Biden with tax crimes dating back to at least 2019.

A federal grand jury in Delaware led by a Trump-picked state attorney general has been examining the younger Biden's taxes and foreign business work since 2018.

Last month, US Attorney General Merrick Garland told a Senate hearing he would not interfere with the investigation.

President Biden has said that he has never spoken to his son about his business dealings.

On Thursday, White House spokesman Ian Sams said Mr Biden had vowed to allow the investigation into his son to be conducted "free from any political interference by the White House".

"He has upheld that commitment," added Mr Sams.