The International Criminal Court on Wednesday ordered the release of the first suspect it took into custody, saying he cannot get a fair trial because prosecutors are withholding evidence.

The order to free Congolese militia leader Thomas Lubanga was a major blow to prosecutors. His trial was to be the first at the court focusing solely on the use of child soldiers.

Prosecutors immediately filed an appeal, which will keep Lubanga in custody for at least five days while the court decides what to do next.

His landmark trial was to have been the first at the world’s first permanent war crimes tribunal, but the proceeding was suspended before it began last month after the prosecution refused to release documents provided by the United Nations that could help clear Lubanga.

He is charged with conscripting and sending children under age 15 to fight in bloody conflicts in Congo’s Ituri region in 2002-03.

In their order, a three-judge panel said that “in the absence of the prospect of a trial, the accused cannot be held in custody.” They added that as the situation stood, “a fair trial of the accused is impossible, and the entire justification for his detention has been removed.”

In appealing against his release, prosecutors said Lubanga could still get a fair trial and urged that the proceeding go forward.

“The Prosecutor strongly believes that the need to do justice for the victims in ... Congo and to respect a fair trial for Thomas Lubanga can and must be harmonized,” prosecutors said in a statement. “The ICC is a permanent and global institution. It must be based on the highest standards of law.”

Chief prosecutor Luis Moreno-Ocampo expressed confidence that Lubanga would stand trial at the Hague-based court.

Other cases currently under way at the court could run into the same kind of turmoil if prosecutors in those cases are relying on similar confidential documents.

“Our understanding is that the problem might be pervasive,” said Geraldine Mattioli of Human Rights Watch. “This is a practice that was pretty widespread at the beginning of the court’s operations.”

The United Nations gave prosecutors many documents only on the condition they remain confidential.

In a letter read at a hearing last week, U.N. Undersecretary-General for Legal Affairs Nicolas Michel offered to let judges look through the documents, but only in a closed room in the presence of a U.N. official. They would be barred from making copies or taking notes, but could jot down “their thoughts” after leaving the room, the letter said.

Presiding judge Adrian Fulford rejected that offer as unworkable.

Michael Scharf, a law professor at Case Western Reserve University who has studied international war crimes tribunals, predicted the U.N. will make a “more reasonable” proposal “that will be accepted by the judges before Lubanga is ever set free and that Lubanga’s trial will proceed after this matter is resolved.”

“The U.N. knows the stakes are far too high not to try very hard to come up with an acceptable compromise with the judges,” he said via an e-mail.

Prosecutors acknowledge that about half of the more than 200 documents they are withholding contain evidence that could help support Lubanga’s innocence or mitigate his guilt.

Lubanga is founder of the Union of Congolese Patriots and prosecutors charge he swelled the ranks of its armed wing with child soldiers at a time when thousands of people were being killed, maimed or raped in brutal ethnic warfare.

Lawyers representing victims of atrocities in Congo warned in court last week that releasing Lubanga without a trial could unleash “a fireball” in the Ituri region where his support is based.

The three-judge panel said it took those fears into account when making the decision.

The court is also dealing with cases involving atrocities in Darfur, the Central African Republic and Uganda, but only has suspects from Congo in its custody.