The US justice department is seeking a top federal judge to hold Donald Trump’s political office in contempt of court for not fully complying with a grand jury subpoena issued in May demanding the return of all classified documents in its possession, according to a source familiar with the matter.

The department in recent weeks asked the chief US district court judge for the District of Columbia, Beryl Howell, to hold Trump’s office in contempt after prosecutors were unable to get the former president’s lawyers to designate a custodian of records to certify all records were returned.

Howell has not ruled on the matter, which remains under seal. But the move, earlier reported by the Washington Post, significantly raises the stakes for Trump as he stares down a criminal investigation into unauthorized retention of national security information and obstruction of justice.

The issue is to do with the Trump legal team’s reluctance to designate a custodian of records to certify that Trump is no longer in possession of any documents marked classified and thus in compliance with the subpoena that demanded the return of all such government records, the source said.

If the Trump legal team could not find someone to certify under oath that all documents bearing classified markings had been returned, the department is said to have communicated, it would seek a judicial sanction.

The contempt action is understood to be focused on Trump’s office because the subpoena, issued on 11 May, sought the return of all documents and writings “in the custody of Donald J Trump and/or the Office of Donald J Trump” bearing classification markings.

In response to the subpoena, the Trump lawyer Evan Corcoran conducted a search of the former president’s Mar-a-Lago resort in Florida and identified a number of pertinent documents, and got another Trump lawyer Christina Bobb to sign a caveated certification certifying all records were returned.

The certification letter, though, was heavily caveated and Bobb insisted on changes to the letter drafted by Corcoran so that it ultimately read she was making the attestation “based on the information provided to me” and “to the best of my knowledge”, the Guardian previously reported.

In the weeks after the FBI seized 103 documents marked classified when officials searched Mar-a-Lago on 8 August, the justice department told Trump’s lawyers that they believed Trump was still in possession of additional documents, and sought a second assurance that no documents were left.

The department never got a second attestation and recently moved to have Trump’s office held in contempt, catching by surprise Trump’s legal team which had decided to take a more cooperative approach with federal prosecutors after initially trying an aggressive approach, the source said.

That appears to have deeply frustrated the government, which told Trump’s lawyers that if they refused to designate a custodian of records to sign a sworn statement attesting that all documents marked classified had been returned, it would formally seek to hold them in contempt.

Should Howell hold Trump’s office in contempt – a closed-door hearing is scheduled at the US district court for the District of Columbia for Friday – it would likely be subject to some form of sanction until the former president’s office is deemed to be in compliance with the May subpoena.

“Contempt is used as a coercive tactic,” said Barbara McQuade, former US attorney and University of Michigan Law School professor. “When it’s an entity, it’s often a monetary fine.”

The impending court battle between the justice department and Trump’s lawyers comes after it emerged that a search of a storage unit in Florida holding boxes of material belonging to Trump turned up two more documents marked classified, in addition to the 103 found at Mar-a-Lago by the FBI.

It was not clear whether the department initiated the contempt proceeding before or after the two additional documents were found. The Trump legal team is understood to have turned over the two new documents as soon as they were discovered, the source said.