Bradley Manning, the US soldier accused of being the source of the biggest leak of state secrets in American history, has won a partial victory in his battle to force the government to disclose vital information that could help his defence.

The judge presiding over his trial at Fort Meade in Maryland has ordered the US government to hand over several confidential documents relating to the massive leak to the whistleblower website WikiLeaks.

In particular, the Obama administration must now disclose to Manning's lawyers some of the damage assessments it carried out into the impact of the leak on US interests around the world.

Should those assessments reveal that the US government found that the fallout from WikiLeaks was limited, that could be used by Manning's defence to argue his innocence against some of the charges he faces, such as aiding the enemy. If the soldier is found guilty, the information might then prove invaluable in reducing any sentence.

As a result of the ruling, Manning's defence team was handed the main findings of a state department investigation into the impact of WikiLeaks on Tuesday evening.

Though the information has not been made public, it is likely to include the assessments of embassies across the globe of the effects on their work of the publication of hundreds of thousands of diplomatic cables.

In addition, Manning's defence lawyers will now also be able to see a redacted report into WikiLeaks by the defence intelligence agency. It was also revealed that the FBI carried out its own inquiry into the leak of confidential material to WikiLeaks, which the Manning's defence lawyers will also now pursue.

News of the breakthrough over the damage assessments came in the first of three days of pre-trial hearings at Fort Meade. The proceedings are being attended by Manning, who sits in full military uniform flanked by his civilian lawyer David Coombs and two military defence lawyers.

The ruling marks an important legal victory as well as a confidence boost for Manning as he approaches a full court martial in September. The army private, who was arrested two years ago outside Baghdad, faces 22 charges with a maximum sentence of life in military custody. It also vindicates Manning's faith in Coombs, who has conducted a robust defence against seemingly endless prevarication and sleight of hand on the part of the military prosecution.

Coombs on Wednesday made a spirited appeal to the court for an end to what he called the government's attempt to play "hide the ball". Coombs protested that whenever he asked the government for specific information he was told he was being too broad. He gave one example of having requested documents emerging from a review carried out by the House of Representatives oversight committee under its chairman, Darrell Issa.

"Short of telling the government that the documents are in a red file in Darrell Issa's third drawer, beneath his Bible, you can't get much more specific than my request," Coombs said.