The military judge in the 9/11 tribunal has ordered the disclosure of correspondence between the International Committee of the Red Cross and the US government about conditions at the Guantánamo Bay detention facility.

The extraordinary ruling, issued by Col James Pohl on Tuesday at Guantánamo, represents a rare opportunity for disclosure of the assessments of the only human rights group with access to the detention facility since 2002.

Pohl’s ruling comes expressly against the wishes of the Red Cross, which argued at Guantánamo in June that its communications with governments enjoy an “absolute privilege”, placing them beyond the purview of the military commission currently preparing to hear the fate of the 9/11 defendants.

The document trove “is the only independent historical record” of what has taken place inside Guantánamo prison walls, said defense counsel James Connell, a Defense Department civilian.

But there will be no immediate public access to nearly a decade of ICRC documentation at Guantánamo.

Pohl will be the first person to see the trove, which he ordered produced to him by December 2, and he will make a determination afterwards on whether the prosecution or defense counsels in the 9/11 trial may view them.

Pohl issued his ruling in response to a series of pre-trial motions by the defense that seek, as he wrote on Tuesday, “to develop potential mitigation evidence and to ensure the conditions for detention are in compliance with international law agreements and standards”. Such evidence is relevant to the defense’s contentions of illegal pre-trial confinement, which is sure to become an issue during sentencing in the capital case should the defendants be found guilty.

"Assuming the defense gets some information out of this, which not been decided but where we hope it’s going, we’ll use that in crafting our arguments about pretrial punishment,” Connell said.

The ICRC argued to Pohl in the spring that there was an “absolute right to non-disclosure of the ICRC's confidential information.” The humanitarian organization has a blanket position that publication of its work may jeopardize its access, granted by governments around the world, to care for detainees and prisoners who might otherwise be disappeared or abused.

Pohl rejected that argument, drawing a distinction between phases of trial.

Civilian courts in the US “have protected ICRC reports from public disclosure under the Freedom of Information Act,” Pohl wrote, “but none have concluded there is privilege from discovery in a criminal proceeding,” referring to a trial phase concerning the production of evidence.

Pohl’s ruling also goes far beyond the production of ICRC correspondents regarding the five Guantánamo detainees accused of masterminding the September 11 attacks, none of whom were at Guantánamo before 2006.

The colonel ordered “all correspondence between the United States and the ICRC, in the possession of the United States pertaining to the ICRC inspections of, and work at, the detention facilities at the US Naval Station, Guantánamo Bay, Cuba” produced to the court.

It is unknown how large a document collection the ICRC and the US government have amassed over a decade of work at Guantanamo.

“I suspect it’s quite large,” said Connell.

“I wouldn’t begin to guess the amount of correspondence that’s involved,” said Lt Col Todd Breasseale, a Pentagon-based spokesman for detention issues.

Pohl’s ruling appears to have caught many of the relevant players offguard. "The ICRC takes note that Judge Pohl did not find an absolute privilege of non-disclosure under US law for ICRC materials," said Anna Nelson, a spokeswoman for the ICRC in Washington. "However, his decision does not mean that ICRC information is unprotected nor does it result in a public release of this information."

"Judge Pohl has undertaken to review the material in camera, under seal, which the ICRC views as an indication of the seriousness with which the Military Commission is treating it," Nelson said.

“As we have only just received the order, the Department is not currently in a position to render a fully informed contemplation of what the judge's order actually demands,” Breasseale said.

The next-pretrial hearing at Guantánamo in the September 11 case is slated for December 16. The trial is not expected to begin until sometime in 2014.