The New Democrats are renewing their call for a public inquiry into the handling of Afghan detainees in 2006-07 after a group of former ambassadors released an open letter Tuesday that criticizes the government's treatment of a senior diplomat.

The Conservative government has denied allegations from Richard Colvin, a former senior diplomat with Canada's mission in Afghanistan, that it knew detainees were being tortured by Afghan officials. (CBC)

Twenty-five former ambassadors have signed the letter, which says that Richard Colvin, a career diplomat who was posted in Afghanistan from 2006 to 2007, was "unfairly subjected to personal attacks" as a result of his testimony about Afghan detainees.

Paul Dewar, the NDP foreign affairs critic, said the government dragged Colvin's reputation through the mud after he told a Commons committee in November that all detainees transferred by Canadians to Afghan prisons were likely tortured by Afghan officials.

Colvin also told the committee that his concerns were ignored by top government officials.

After Colvin's testimony, Defence Minister Peter MacKay painted him as having been duped by the Taliban and said Canadians were being asked to accept the word of prisoners.

Dewar said MacKay should apologize for the way he tried to discredit Colvin and resign his post as defence minister.

'While criticism of his testimony was perfectly legitimate, aspersions cast on his personal integrity were not.'—Letter from ex-diplomats

"If Mr. MacKay is not willing to apologize and offer his resignation, then I think the prime minister should demand it," Dewar said.

The NDP also said MacKay misled the House of Commons at least nine times when he said there was not a single proven case that a detainee was abused when handed over to Afghans by Canadian.

Recent media reports have suggested that there is at least one documented case of detainee abuse, but Gen. Walter Natynczyk, Canada's chief of defence staff, told the House of Commons defence committee Tuesday that Canadian troops questioned — but didn't detain — the suspect who was reportedly tortured.

Diplomats must be able to report honestly, letter says

"While criticism of his testimony was perfectly legitimate, aspersions cast on his personal integrity were not," the letter says.

The letter says foreign service officers must be able to report on a situation as it is "observed or understood."

"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," the former diplomats said in the letter.

Commission of inquiry may be needed, ex-diplomat says

Gar Pardy, a former head of Canadian consular services and a former ambassador to Central America, said the letter takes issue with the government's response to Colvin's testimony.

"The Commons committee invited him to give testimony, and Mr. Colvin showed up, gave testimony, and immediately after, ministers of the Crown decided to attack his credibility, his honesty and his professionalism, and we take issue with that," Pardy said Tuesday.

Public servants must be able to provide honest and professional advice to the government, Pardy said.

"If they're prevented from doing that because of the atmosphere in Ottawa, we've lost something very important as far as the government of Canada is concerned," he said.

Pardy said he expects more former diplomats will come forward and sign the letter.

With files from The Canadian Press