The special counsel investigating Hunter Biden has charged a former F.B.I. informant with fabricating claims that President Biden and his son sought two $5 million bribes from a Ukrainian energy company, according to an indictment in a California federal court.

The former informant, Alexander Smirnov, 43, was accused of falsely telling the F.B.I. that Hunter Biden, then serving as a paid member on the board of Burisma, demanded the money to protect the company from an investigation by the country’s prosecutor general at the time.

The story Mr. Smirnov told investigators was part of a series of explosive and unsubstantiated claims by Republicans that the Bidens engaged in potentially criminal activity — allegations central to the party’s efforts to impeach the president.

In July, Senator Charles E. Grassley, Republican of Iowa, released a copy of an F.B.I. record that included the false allegation without naming Mr. Smirnov, or questioning its veracity.

“I’ve been pushing the Justice Department and F.B.I. to provide details on its handling of very significant allegations from a trusted F.B.I. informant implicating then-Vice President Biden in a criminal bribery scheme,” Mr. Grassley said in a statement at the time, praising “heroic whistle-blowers” who brought the document to light.

The claims in the report turned out to be a lie, said the special counsel, David C. Weiss, the Trump-appointed U.S. attorney in Delaware.

Mr. Smirnov now faces a two-charge indictment for making false statements and obstructing the government’s long-running investigation of the president’s troubled son. If convicted, he faces a maximum penalty of 25 years in prison.

Mr. Smirnov is a permanent resident of the United States, according to a senior law enforcement official. He was arrested in Las Vegas on Wednesday after disembarking from an international flight and was expected to appear before a federal judge later Thursday.

Hunter Biden promised to protect the company “through his dad, from all kinds of problems,” Mr. Smirnov falsely claimed to the bureau in 2020, according to Mr. Weiss, the special counsel who has charged the president’s son twice over the past year on tax and gun charges.

Mr. Smirnov, prosecutors working for Mr. Weiss said, was only in contact with Burisma executives in 2017, after Mr. Biden left office — when he “had no ability to influence U.S. policy.”

He is accused of exaggerating his “routine and unextraordinary business contacts with Burisma” into “bribery allegations” against the president, who is identified in the filing as “Public Official 1.”