OTTAWA - Call it "taking the 38th."

Invoking the U.S. Fifth Amendment -- taking the Fifth -- made its way into popular culture decades ago. But a distinctly Canadian twist on stonewalling questions under oath will be on prominent display as the country's military police complaints commission resumes public hearings Wednesday.

The 38th refers to Section 38 of the Canada Evidence Act, a national security clause that federal lawyers are expected to call into effect when witnesses are asked to testify on what Canadian police knew, or should have known, about the alleged torture of Afghan prisoners.

Although different in intent and application, the effects of Canada's Section 38 and the U.S. Fifth Amendment -- which protects against a witness's self-incrimination -- are the same.

Both erect a legal shield to deny the release of information.

Defence Minister Peter MacKay assured the House of Commons on Tuesday that the Conservative government is co-operating with the commission, despite a flurry of legal challenges and the enforced departure of the commission's chairman.

He pledged that witnesses will be able to testify "within the mandate of the commission." But what he didn't say was even those statements will be subject to Section 38, which prescribes five years in prison for those who are deemed to violate national security.

NDP Leader Jack Layton said he's concerned the long-standing provision, strengthened when Ottawa beefed up anti-terrorism laws in 2001, is being used as a blanket to smother the abuse allegations.

"I think the government is using a section (of the law) it has decided to interpret in the broadest possible fashion and it will prevent Canadians from hearing the truth about some very disturbing incidents," said Layton.

Federal lawyers don't have to stretch too much.

The clause is sweeping and gives the justice minister wide discretion to withhold information that could be "injurious to international relations or national defence or national security."

Any government employee even thinking of disclosing information they believe might be sensitive must seek permission in writing ahead of time and the attorney general has the right to issue a certificate to prevent the disclosure of information.

Aside from using the clause to keep a tight leash on those witnesses who will appear before the police commission, federal lawyers have apparently been using it as a hammer against anyone inside the government within reach of the Afghan prisoner file.

"It has been very ominous and intimidating," said a defence source.

Wielding the national security clause came as something of a surprise to those involved in the police commission process. Up until recently, federal lawyers had played tough but had insisted that defence, foreign affairs and development staff co-operate fully with the investigation.

That inexplicably changed this past summer when government lawyers began throwing around Section 38 and its ramifications, as well as trying to shut down pre-inquiry interviews with witnesses, said one source close to the process.

Errol Mendes, a human rights lawyer at the University of Ottawa, said the government's use of the clause became absurd when federal lawyers tried to impose national security prohibitions on publicly available documents.

Mendes called the Canada Evidence Act "one of the most important laws protecting the safety and security of this country and the people who live in it.

"How appropriate is it to be using (Section 38) in a way like a sledge hammer, flinging it around in all directions?" he asked.

"It potentially undermines the rule of law in this country."