The testimony of a Canadian diplomat before a parliamentary committee Wednesday is likely to provide disturbing information about the government's handling of Afghan detainees, CBC News has learned.

The testimony of diplomat Richard Colvin is expected to provide details of what sources describe as an "unusual system" that saw Afghan detainees transferred to Afghan prisons, with little care about the conditions there.

"I think it will be a difficult story for Canadians," a source told the CBC, adding they could be both surprised and disturbed by what Colvin says.

Opposition MPs are expected to focus their questions to Colvin on what the government knew about the alleged abuse of Afghan detainees and what it chose to do about it.

Colvin worked in Kandahar for the Department of Foreign Affairs in 2006. He later moved to Kabul, where he was second in command at the Canadian Embassy. In both jobs, Colvin visited detainees transferred by Canadian soldiers to Afghan prisons. He wrote reports about those visits and sent them back to Ottawa.

A source familiar with the handling of detainees during the time Colvin was in Afghanistan, said the diplomat could reveal ugly information about the way the government responded to the reports.

The government could end up accused of being simply unwilling to deal with the torture claims or of "looking the other way," as the source put it.

In an affidavit filed before the Military Police Complaints Commission, Colvin said he told officials in Ottawa about allegations of abuse and torture of Afghan detainees as early as May 2006. At the time, the government denied there were any credible allegations of torture.

Colvin's first memo on the issue was sent to senior military and foreign affairs officials and described what he thought were "serious imminent and alarming" problems in the handling of detainees.

Colvin wrote at least 16 more memos about the detainee issue, over the next year and a half. In October, CBC News reported the contents of one of those memos, written by Colvin in June 2007.

The report said one detainee claimed to have been beaten with cables and wires and given electric shock. It said another detainee also claimed he was beaten, and forced to stand for two days during interrogation.

The report was widely circulated at the senior levels of the Defence and Foreign Affairs departments. It was also sent to one of Prime Minister Stephen Harper's senior security advisers.

According to a report Tuesday by The Canadian Press, it was at about this time that Canadian diplomats in Afghanistan were ordered to hold back information in their reports to Ottawa about the handling of detainees.

The Canadian Press report quotes defence and foreign affairs sources who say the instruction was aimed at defusing the explosive human-rights controversy emerging in Ottawa at the time.

The sources spoke under the condition of anonymity. The sources told the news agency there was a fear that graphic reports, even in censored form, could be uncovered by opposition parties and the media through access-to-information laws, leading to revelations that would further erode already-tenuous public support for the Afghan mission.

The controversy was seen as "detracting from the narrative" the Harper government was trying to weave around the mission, said one official.

The instruction was passed over the telephone by senior officials in the Privy Council Office and reinforced in followup conferences between Ottawa and Kabul, as well Ottawa and Kandahar, The Canadian Press reported.

Liberal Defence Critic Ujjal Dosanjh says Colvin's testimony Wednesday could show that senior members of government, and perhaps even the prime minister, knew there was a risk of torture in Afghan prisons but still allowed detainees to be transferred.

"Mr. Colvin is the central figure," Dosanjh told the CBC in an interview. "He is the one who says he wrote about the allegations of torture, wrote about the existence of torture at the hands of the Afghan authorities and wrote to Canadian diplomats and others.

"It is my contention that if this matter is properly unearthed and investigated you would see that the knowledge of these allegations of torture go right up to the prime minister and various ministers, at that time."

With files from The Canadian Press