A federal judge on Wednesday rejected a bid by the Justice Department to free a former F.B.I. informant who had pleaded guilty to lying about Hunter Biden and evading his taxes, saying that nothing about the facts of the case had changed and the man might still flee if released.

The longtime informant, Alexander Smirnov, pleaded guilty in December in exchange for a six-year prison sentence, admitting that he had lied to the government when he claimed to have information about a multimillion-dollar bribery scheme involving President Joseph R. Biden Jr. and his son Hunter.

Before Mr. Smirnov was charged and eventually admitted his guilt, Republican lawmakers had promoted his false claims about the Bidens in their push to try to impeach President Biden. During the 2024 presidential campaign, Mr. Smirnov’s allegations were also amplified by the Trump supporter Kash Patel, who is now the director of the F.B.I.