Democrats control Congress and its committees, making such a request from Republicans virtually impossible, though mid-term congressional elections could shift control. The request to Treasury was made under the Bank Secrecy Act which is intended to help prevent money laundering.

Representative James Comer of Kentucky, the top Republican on the House Oversight and Reform Committee, has accused the Treasury of changing rules to shield Hunter Biden's business dealings with foreign companies.

Former President Donald Trump and his Republican allies in Congress made Hunter Biden’s business dealings in China and Ukraine a line of attack against the elder Biden during the 2020 election campaign. Hunter Biden has denied any wrongdoing.

In July, Comer said Treasury officials had told the committee's Republican staff that the department would not provide access to the suspicious activity reports unless Democrats join the request.

Suspicious activity reports are filed by financial institutions when clients make large cash transactions or transfers above $5,000 that could signal money laundering or other offenses, although many such transactions are legitimate.

In a letter to Comer seen by Reuters, the Treasury said such reports are normally kept confidential but that it complies with current statutes and regulations on providing requested information to Congress. Such access would require written requests from committees, but Treasury Secretary Janet Yellen has the final say.

"Under the current regulation, the Secretary may make BSA information available to 'Congress, or any committee or subcommittee thereof, upon a written request stating the particular information desired, the criminal, tax or regulatory purpose for which the information is sought, and the official need for the information,'" the Treasury wrote. "These decisions are entrusted to the Secretary's discretion."

Comer said in a statement that it was "unacceptable" for the Treasury to withhold the reports and added that the department was "running cover" for the Biden family.

"We need more information about these transactions and if Joe Biden has benefited financially from his family’s dealings with foreign adversaries," Comer said. "Republicans will use the power of the gavel next Congress to get them."

While House Republicans currently in the minority do not have authority to make formal committee requests for such reports, including for Hunter Biden, the tide could shift if they win control of the House in November congressional elections.

Treasury said a requesting committee should provide a detailed statement of purpose for seeking the information to ensure that it meets Bank Secrecy Act purposes and protecting law enforcement investigations.

"It is not a political process," a Treasury official said in a statement. "Since the beginning of this Administration, Treasury has made SARs available in response to authorized committee requests and continues to engage on the process with any individual members seeking information."

Similar notices have previously been sent to Democratic lawmakers requesting suspicious activity reports.

The Treasury said that when it approves requests for Bank Secrecy Act information to any authorized party, it is provided only in secure reading rooms that are meant to keep the information confidential.