CNN– In a monumental ruling Friday, a federal judge ordered Donald Trump attorney Evan Corcoran to provide additional testimony as part of an investigation into the former president’s handling of classified documents, a source familiar with the matter told CNN.

Corcoran has the potential to become one of the most crucial witnesses in special counsel Jack Smith’s criminal investigation into possible mishandling of classified records after the Trump presidency and obstruction of justice.

District Judge Beryl Howell said in an order under seal that Justice Department prosecutors have met the threshold for the crime-fraud exception for Corcoran, the source said.

The scope of what DOJ will be allowed to ask, however, was not immediately clear. Trump’s team is expected to appeal and ask for the judge’s order to be stayed while legal proceedings play out.

The decision hands Trump yet another loss under seal in court as his team and allies have tried to hold off Smith’s investigators from learning about direct conversations the former president had with some of his closest advisers.

The development is particularly notable because of accusations prosecutors would have made about Trump as they argued to the judge for the grand jury testimony.

A Trump spokesperson said the ruling is a “violation of due process.”

“Whenever prosecutors target the attorneys, that’s usually a good indication their underlying case is very weak. If they had a real case, they wouldn’t need to play corrupt games with the Constitution. Every American has the right to consult with counsel and have candid discussions – this promotes adherence to the law,” the spokesperson said.

Corcoran, an attorney-turned-witness, had previously testified to the grand jury but declined to answer some questions, citing attorney-client privilege. The department argued to the judge he should not be able to avoid answering, because his discussions with the former president may have been part of an attempt to plan a crime.

Howell’s ruling is one of her last on sealed grand jury disputes as chief judge. The Obama appointee has repeatedly green-lit Justice Department requests to pursue information about Trump’s actions during her tenure as chief of the DC District Court, but she rotates out of the administrative role on Friday.

When Corcoran first testified to the grand jury in January, he was asked about what happened in the lead-up to the August search of Trump’s Mar-a-Lago residence.

Corcoran had drafted a statement in June that attested Trump’s team had done a “diligent search” and there were no more classified documents at Trump’s Florida residence.

After that, the FBI searched Mar-a-Lago and found hundreds of government records, including classified material, raising questions about the lawyer’s attestation.

This story has been updated with additional details.

CNN’s Paula Reid contributed to this report.s