A federal judge on Monday intervened in an investigation into former President Donald J. Trump’s handling of sensitive government records, ordering the appointment of an independent arbitrator to review a trove of materials released last month from Mr. Trump’s private club and residence in Florida were confiscated.

In a 24-page ruling, Judge Aileen M. Cannon of the Federal District Court for the Southern District of Florida also barred the Justice Department from using the seized materials for “investigative purposes” related to Mr. Trump’s ongoing investigation pending the arbitrator’s work , known as the Special Master, was completed.

The order would effectively bar federal prosecutors from using a key piece of evidence while they continue to investigate whether the former president unlawfully kept national defense documents at his Mar-a-Lago estate or impeded government efforts to get them back.

In her order, issued on the Labor Day holiday, Judge Cannon said she made her decisions “to ensure at least the appearance of fairness and integrity in the exceptional circumstances.” However, their order would not affect a separate review of the documents by the Office of the Director of National Intelligence.

Judge Cannon’s decision gave the Special Master sweeping powers to review materials extracted from Mar-a-Lago, some of which bore markings identifying them as top secret. It allowed anyone eventually appointed to the post to assess the documents not only for those protected by attorney-client privilege, a relatively common measure, but also for those potentially protected by executive privilege, the normally confidential internal Executive branch deliberations protects.

At a hearing on the issue last week, the Justice Department argued that since Mr Trump is a former President and the Department is itself, allowing a special master to conduct an executive privilege review of the seized material would be a radical and legally unfounded move Part of the current executive branch.

But Judge Cannon disagreed with the Justice Department, writing in the order that she was “unconvinced” by the government’s categorical claim that executive privilege did not apply in this context. She added that she felt the department’s position “arguably exaggerated the law” and that it made sense for her to set aside any documents that might be protected by executive privilege if the legal issues in the case are resolved.

“Even if any assertion of executive privilege by plaintiff in this regard ultimately fails, that possibility, even if probable, does not negate a former president’s ability to assert the privilege as a matter of first concern,” she wrote. “Because the eligibility review team did not search for any material that may be subject to executive privileges, further review is required for this additional purpose.”

A Justice Department spokesman did not initially respond to a request for comment, but Department officials last week discussed the possibility of an appeal should the judge rule in Mr Trump’s favour.

Glenn Thrush contributed reporting.