A legal brief that exempted the US military from criminal laws following the 9/11 attacks was improperly kept classified for years, the former head of the US government agency in charge of document secrecy said today.

The March 2003 brief, which allowed Pentagon interrogators to claim self-defence in sidestepping laws against torture, was made public earlier this month.

J William Leonard, who directed George Bush's information security oversight office until last year, today told Congress that the document never should have been classified in the first place.

"To learn that such a document was classified had the same effect on me as waking up one morning and learning that after all these years, there is a 'secret' article to the constitution that the American people do not even know about," Leonard said.

The former classifications chief appeared at a hearing that focussed on the still-secret opinions of the office of legal counsel (OLC), a little-known arm of the US justice department that provides the White House with legal advice.

The 2003 brief, written by former OLC lawyer John Yoo, lies at the centre of a furious row over the legality of violent interrogations approved by the Bush administration at Guantanamo Bay and secret "black sites" run by the CIA.

Democratic senator Russell Feingold, who chaired today's hearing, revealed that he and some colleagues had reached a deal with Bush aides to receive limited access to secret legal briefs on interrogations.

John Elwood, an OLC lawyer who testified today, could not say whether congressmen would be able to view all the briefs they sought. Instead, Elwood attempted to convince Feingold that Congress does not need to see OLC opinions in order to respond to them.

"Just because we don't turn over our communications to our client [the president] … doesn't mean you can't know what our position on the law is," Elwood said.

He added that OLC opinions do not necessarily provide a defence against liability in court. Such an argument could expose senior Bush aides, who relied on OLC's advice in using brutal tactics on terrorist suspects, to civil lawsuits in coming years.

The March 2003 legal brief was made public thanks to a lawsuit filed by a non-profit activist group, the American Civil Liberties Union. Feingold and fellow Democrats have unsuccessfully pushed for the release of a 2001 memo, cited in that brief, suspending part of the constitution during military operations after 9/11.

While Leonard said the classification of the 2003 brief demonstrated "profound ignorance of or deep contempt for" rules on government secrecy, he said the decision might have been made to "keep it out of the hands of the military services' legal people".

If Yoo's legal justifications for the use of torturous tactics were openly shared with the military, American troops would know they faced a similar risk of violent treatment if captured in Iraq or Afghanistan, Leonard said.

Leonard's explanation tracks with the conclusions of Philippe Sands, one of Britain's top human rights lawyers. In his latest book, Torture Team, Sands found US military lawyers maintaining back-channel contacts with the justice department officials who crafted the 2003 brief.

Sands is slated to testify before the judiciary committee of the House of Representatives on interrogation policy next week.