Washington CNN  — 

Hunter Biden’s lead criminal defense attorney asked a federal judge on Tuesday for permission to withdraw from the case because he could now be called as a witness in future proceedings.

Christopher Clark, Biden’s longtime defense lawyer, filed a motion with the Delaware judge who has overseen the case since a plea deal was announced in June.

“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,” Biden’s lawyers said in the filing.

Federal prosecutors said last week they reached an impasse on a plea deal related to tax offenses and a related “diversion agreement” to resolve a gun possession charge. They asked the judge to withdraw a late August deadline she previously gave both sides to re-negotiate the agreements after she refused to approve them at a hearing last month.

Biden’s lawyers said Sunday they believed the gun diversion deal was still “valid and binding.” Abbe Lowell, one of the attorneys, placed blame on prosecutors for drafting the agreements the judge took issue with.

The judge has asked for further information and has not ruled on the status of the case.

As the plea talks fell apart last week, the prosecutor handling the case, US attorney David Weiss, requested and was granted “special counsel” status by Attorney General Merrick Garland.