The controversy over the Afghan detainee issue rose up anew Tuesday as an unprecedented letter from 23 former ambassadors condemning the Harper government led to calls for Defence Minister Peter MacKay's resignation.

The opposition called for MacKay to step down, saying he had misled the House of Commons on what the government knew about the possible torture of Afghan detainees being handed over by Canadian soldiers to local authorities.

"MacKay has zero credibility," MP Paul Dewar told a news conference. "The buck stops with MacKay and he has to go, and the prime minister must call a public inquiry into this cover-up."

Liberal defence critic Ujjal Dosanjh added he had "confidence in the military but I have no confidence in that minister right now."

"The Conservatives refuse to tell the truth on detainees," he said. "They censor documents. They intimidate public servants."

The issue, which had taken a political backset over the last week, came back in a flurry Tuesday after a letter signed by 23 ex-ambassadors slammed the Conservative government for attacking the credibility of diplomat Richard Colvin.

"Colvin, a Foreign Service Officer dedicated to discharging his responsibilities to the best of his ability under difficult circumstances, was unfairly subjected to personal attacks as a result of his testimony," the letter reads. "The Colvin affair risks creating a climate in which Officers may be more inclined to report what they believe headquarters wants to hear, rather than facts and perceptions deemed unpalatable."

One of the diplomats who signed the letter was Robert Fowler, who recently spent five months in captivity in Niger when he was working as a special envoy for the United Nations.

Jacques Roy, a former ambassador to France, told CTV New Channel's Power Play that Colvin was summoned to give his testimony in front of a parliamentary committee and was "doing his job as well as he could."

"Our diplomats abroad should continue to report as faithfully as they can," he added.

Garfield "Gar" Pardy, former head of Canadian Consular Services, said that the government has unfairly attacked Colvin, especially considering ministers are supposed to be responsible for their department.

"Officials play a particular role in our system of government and so do ministers -- ministers are the ones who are supposed to stand up and take the heat if things go wrong," he told Power Play.

Conservatives downplayed the letter, saying the majority of foreign services personnel did not sign it.

"(But) it's not a number that's insignificant," Laurie Hawn, the parliamentary secretary for to the Defence Minister, said of those who signed the letter on Power Play. "But the fact is, Richard Colvin was free to report, and we are free to disagree with his message.

"Just because we disagree with the message, doesn't mean we are shooting the messenger."

Earlier in the day, Hawn was heard uttering "bullshit" in the House of Commons, as the government took questions from the Bloc Quebecois on the Afghan detainee issue.

The government has argued there was no evidence Canadian-captured detainees were being abused prior to a new transfer agreement in 2007.