Canada's chief of defence staff said Sunday the practice of transferring detainees from Canadian to Afghan custody has been stopped on more than one occasion over safety concerns.

Gen. Walt Natynczyk did not say when the transfers were halted or how many times.

But it's a staggering admission given the charges levelled this week by intelligence officer and former diplomat Richard Colvin, who told a Commons committee this week that likely all detainees Canadian forces handed over to Afghan officials in 2006 and 2007 were tortured.

Colvin's allegations appear to be corroborated by an April study from the Afghanistan Independent Human Rights Commission. A translation of the study obtained by The Canadian Press found 47 cases of abuse in Kandahar, which had the third-highest incidence of abuse claims in the country.

"Torture and other cruel, inhuman or degrading treatment are common in the majority of law enforcement institutions, and at least 98.5 per cent of interviewed victims have been tortured," the study said.

Abuse claims were recorded between 2001 and early 2008. Most took place in 2006 and 2007, when Colvin said he was warning the federal government about torture in Afghanistan.

The Conservative government has acknowledged that the Canadian Army halted the practice of transferring detainees to Afghan custody due to torture concerns on one occasion, in November 2007.

But the issue will likely continue to gain steam this week, when former military commanders, including retired Gen. Rick Hillier, appear before the Commons committee.

Earlier Sunday, the NDP's defence critic said an inquiry "is the only way" to get to the bottom of what the Conservative government knew about allegations of torture among detainees by their Afghan captors.

Speaking to CTV's Question Period Sunday, MP Jack Harris said the government should be ashamed of its attempts to discredit Colvin, who also said that many of the prisoners were likely innocent of any crimes, and that he was told not to mention prisoners in his reports from the field.

Colvin was the political officer at the Canadian-run provincial reconstruction base when Canadian troops began transferring prisoners to Afghan authorities three years ago.

Defence Minister Peter MacKay and other members of Prime Minister Stephen Harper's cabinet questioned Colvin's claims, saying there was no evidence to support them and even calling him a "suspect source."

"This government has been trashing this guy as if he's some sort of Taliban dupe. I think it's shameful, and the government should be hanging its head in shame as to what they've been doing to this man in the last few days," Harris said.

"But the underlying theme still is that this man was making the reports as early as May of 2006, changes were not undertaken until at least a year later and it's questionable whether they were ever in effect," he said. "So something is going on here and we need to get to the bottom of it and we think an inquiry is the only way."

Errol Mendes, a professor of constitutional and international law at the University of Ottawa, said he was "astonished" that the government questioned the credibility of Colvin's claims, considering that the charges have been made by other organizations.

"If the government is calling his credibility into question, they have to also question the credibility of NATO, of the U.S. State Department, of the UN, of the Afghanistan Independent Human Rights Commission -- just about all of our allies in Afghanistan who have corroborated the fears of Richard Colvin," Mendes told CTV News Channel Sunday.

Mendes said that under international law those in senior civilian and military positions are responsible to take all reasonable steps to stop potential war crimes such as torture.

"Statements are being flung around now that people didn't know there was a substantial risk of torture -- that is a defence under the doctrine of command responsibility under the Geneva Conventions and the International Criminal Court," he said.

The charges

Colvin said that when he learned of the torture allegations, including that prisoners were subjected to beatings and electric shocks, he wrote to officials in Ottawa throughout 2006 and into 2007.

Those reports were widely circulated in the Foreign Affairs and Defence Departments, yet senior members of the Conservative government, including Harper, say they did not see them in 2006.

Globe and Mail reporter Graeme Smith, who broke the story of abuse in Afghan prisons, said Colvin has "torpedoed" his career by levelling charges of illegal activity against government ministers.

"I nearly fell off my chair, to be honest. I didn't expect Colvin to be the kind of guy who would talk in the first place," Smith told Question Period. "Colvin is this very British, stiff-upper-lip, by-the-book, by-the-rules kind of guy."

"For the government to be coming out and calling him a Taliban dupe, is astonishing to me," Smith added. "He works in this highly, highly classified environment, I don't quite understand how he's a Taliban sympathizer."

Laurie Hawn, parliamentary secretary to MacKay, said that there are two issues: the timeline of who knew what and when, and what was happening inside Afghan prisons.

"Clearly Afghan prisons are not like Canadian prisons," Hawn told Question Period.

"But at that time, under the agreements we were operating under, no Canadian soldier or commander, and no minister and no member of the government, knowingly transferred prisoners in to be tortured."

Hawn repeated MacKay's comments from earlier this week, in which he asserted that once the government received "credible information" about poor conditions in Afghan prisons, it made a series of changes to its transfer policy and boosted funding for prison infrastructure.

"And according to all sources that's when things really started to improve and things are much, much better now than they were," Hawn said.

Liberal foreign affairs critic Bob Rae said assertions that the prime minister and others did not know about Colvin's allegations in 2006 are not "credible."

He and other Liberal MPs accused the government earlier this week of trying to quash Colvin's reports and cover-up the fact it did nothing about them.

"It's about what happened to the detainees," Rae told Question Period. "It's also about the fact that it took a long time for the Canadian government to change its policy and to try and deal with the problem... It took a year-and-a-half from the time the first detainees were transferred."

With files from The Canadian Press