The defence team representing Bradley Manning, the US soldier who leaked reams of state secrets to WikiLeaks, has made one last attempt to persuade the judge presiding over his court martial to dismiss the most serious charge against him: that he "aided the enemy".

Manning's civilian lawyer David Coombs said that to convict the army private of such a severe offence would set an "extremely bad precedent". It would place US society on a "very slippery slope, of basically punishing people for getting information out to the press."

Addressing the judge in a military court in Fort Meade, Coombs said that "no case has ever been prosecuted under this type of theory: that an individual, by the nature of giving information to a journalistic organisation, would then be subject to" a charge of "aiding the enemy". Conviction of such an office would bring "a hammer down on any whistleblower or anybody who wants to put information out".

The lawyer's comments were recorded by the Freedom of the Press Foundation, which is employing court stenographers during the Manning trial as a way of overcoming the high level of official secrecy that surrounds the case.

The "aiding the enemy" charge has become the seminal battle in Manning's prosecution, with the US government, in its determination to come down hard on official leakers, ranged against advocates of freedom of information. Under the terms of the charge, Manning is accused of having given valuable intelligence to Osama bin Laden, al-Qaida and its affiliate groups simply by dint of having leaked documents that were then posted by WikiLeaks on the internet.

In a further charge under the 1917 Espionage Act, the soldier is accused of "wrongfully and wantonly" causing intelligence to be published on the internet when he knew that would make it "accessible to the enemy".

Human rights groups are now focusing in on the "aiding the enemy" charge as a threat to free speech in America. Earlier this week Amnesty International derided as "ludicrous" the claim that by leaking to a news organisation an individual could be guilty of helping al-Qaida.

In spirited legal argument, Coombs told the court that the prosecution had entirely failed to show that Manning had any "actual knowledge" that transmitting documents to WikiLeaks would be beneficial to al-Qaida. He said that the webchats that the soldier conducted with the former hacker Adrian Lamo prior to his arrest indicated the opposite: "that his intent was to get this information out to spark reform, spark debate".

Coombs reminded the court that "aiding the enemy" carries the death penalty, and although the prosecution is not pursuing a capital case against Manning, the soldier does face the possibility of life without parole. To find him guilty of such a severe offence, Coombs said, the government would have to meet a high evidential standard by proving that Manning had "actual knowledge" he was giving intelligence to al-Qaida.

But, Coombs said, the government has no such evidence. "The government has nothing, but perhaps an argument that PFC Manning might have been negligent in giving information to WikiLeaks, and that the enemy might have been able to access it. But there has been no evidence offered by the government to show actual knowledge."

Speaking for the government, Captain Angel Overgaard said sardonically that "it would be nice if we had a videotaped confession" from Manning admitting that he knew he was making the leaked documents available to al-Qaida. "We don't have that in this case."

But the prosecutor said that the government had presented the court with "a mountain of circumstantial evidence … that shows that the accused did, in fact, know that by publishing information, leaking information to WikiLeaks, and having it published on the internet, that it was, in fact, going to al-Qaida."

Colonel Denise Lind, the judge presiding over the case alone in the absence of a jury, repeated a question that has come up in earlier pre-trial hearings. "Does it make any difference if it's WikiLeaks or any other news organisation – the New York Times, Washington Post, or Wall Street Journal?"

Overgaard huddled for a moment with her fellow prosecutors, and then replied: "No, it would not. It would not potentially make a difference."

Overgaard said in this case Manning was a trained intelligence analyst, and as such "knew exactly what he was doing". She added: "PFC Manning is distinct from an infantryman or a truck driver, because he had all the training. And this was his job. He knew exactly the consequences of his actions."

The judge has indicated that she will rule later this week on the defence's motion to have the "aiding the enemy" charge dismissed.