Updated 14:30 EDT

The lead criminal defense lawyer representing Hunter Biden for the past two years told a federal judge that he is stepping aside from his job Tuesday, citing the possibility that he may be called as a witness in future proceedings.

Christopher Clark, who overall has represented President Biden’s son for five years, told the Delaware federal judge who has been presiding over the case that Delaware rules state that a lawyer may not advocate at a trial at which he or she is likely to be called as a witness.

“Based on recent developments, it appears that the negotiation and drafting of the plea agreement and diversion agreement will be contested, and Mr. Clark is a percipient witness to those issues,” Mr. Clark told the judge.

The younger Mr. Biden was due to settle his case with the Department of Justice during a hearing in late July, but the deal unexpectedly fell apart at the last minute. Another lawyer working with Hunter Biden, Abbe Lowell, said in a court pleading filed Monday that federal prosecutors “reneged” on the plea agreement at the last minute.

In a television appearance Sunday, Mr. Lowell contended that the plea agreement was still “valid and binding,” but in a filing Tuesday at the Delaware court, the prosecutor named as Special Counsel in the case Friday by Attorney General Garland, David Weiss, refuted that claim. Because the agreement was never signed by a probation officer in the state, he said, the so-called diversion agreement is officially dead.

Because the probation officer “did not approve the now-withdrawn diversion agreement, it never went into effect and, therefore, none of its terms are binding on either party,” Mr. Weiss said.

Mr. Lowell was officially added to the team representing the younger Mr. Biden Monday as part of a reshuffle following the collapse of the plea agreement. Mr. Lowell, who has also represented President Trump’s son-in-law, Jared Kushner, was added as “additional counsel” to the case in a filing Monday morning.

Among Hunter Biden’s myriad lawyers, Mr. Clark was reportedly in charge of fending off the justice department’s investigation into tax fraud and gun law violations. Mr. Lowell was in charge of defending the younger Mr. Biden from the various Republican-led committees in Congress looking into his foreign business dealings and his father’s involvement in those deals.

Bringing Mr. Lowell into the mix was seen by some legal observers as an effort by Hunter Biden and his supporters to take a more aggressive tack against his critics. Since coming on board, he has sent cease-and-desist letters to some of the first son’s most vocal detractors, waded into the debate over Hunter Biden’s famous laptop, and flew to Arkansas to help settle a paternity case against him in that state.

As Mr. Biden’s lead criminal defense lawyer, he would have been intimately involved in the negotiations with the Department of Justice seeking to avoid jail time for the two tax charges and a gun law violation that he faced at Delaware.