The US government is deliberately attempting to prevent Bradley Manning, the alleged source of the massive WikiLeaks trove of state secrets, from receiving a fair trial, the soldier's lawyer alleges in new court documents.

David Coombs, Manning's civilian lawyer, has made his strongest accusations yet about the conduct of the military prosecutors. In motions filed with the military court ahead of a pre-trial hearing at Fort Meade, Maryland, on Monday, he goes so far as to accuse the government in essence of lying to the court.

Coombs charges the prosecutors with making "an outright misrepresentation" to the court over evidence the defence has been trying for months to gain access to through disclosure.

The dispute relates to an investigation by the Office of the National Counterintelligence Executive, Oncix, into the damage caused by the WikiLeaks disclosures of hundreds of thousands of confidential documents.

Reports by the Associated Press, Reuters and other news outlets have suggested that official inquiries into the impact of WikiLeaks concluded that the leaks caused some "pockets" of short-term damage around the world, but that generally its impact had been embarrassing rather than harmful.

Such a finding could prove invaluable to the defence in fighting some of the charges facing Manning or, should he be found guilty, reducing his sentence.

Yet Coombs says the army prosecutors have consistently kept him, and the court, in the dark, thwarting his legal rights to see the evidence.

"It was abundantly clear that Oncix had some form of inquiry into the harm from the leaks – but the government switched definitions around arbitrarily so as to avoid disclosing this discovery to the defence."

On 21 March, the prosecutors told the court that "Oncix has not produced any interim or final damage assessment" into WikiLeaks.

Coombs alleges that this statement was inaccurate – and the government knew it to be inaccurate at the time it made it.

"The defense submits [this] was an outright misrepresentation," he writes.

On 20 April, the government told the court that "Oncix does not have any forensic results or investigative files". Yet a week before that, the prosecutors had handed to the defence documents that clearly showed Oncix had begun to investigate WikiLeaks almost 18 months previously.

"Oncix was collecting information from various agencies in late 2010 to assess what damage, if any, was occasioned by the leaks. So how could it be that Oncix neither had an investigation nor a damage assessment?" Coombs writes.

The alleged efforts by the US government to avoid fulfilling its obligations to hand over evidence, Coombs says, has had the effect of rendering it impossible for the defence to prepare for the trial which is scheduled to begin in September.

Without access to the information, they cannot identify witnesses, develop questions for those witnesses, prepare a cross-examination strategy and so on.

"There is no way that the defense can adequately prepare its case," Coombs complains.

Most damningly, he alleges that is precisely the army's intention. "The government should not be able to circumvent its discovery obligations for two years, then dump discovery on the defense last-minute, and expect that there will be a fair battle," he says.

"Indeed, the defense believes that this was the intention of the government – to defeat its adversary by adopting untenable litigation positions designed to frustrate discovery."

Manning will make his fourth court appearance at Fort Meade on Monday. If convicted of the 22 counts, which include "aiding the enemy", he could be sentenced to spend the rest of his life in military custody.