NDP Leader Thomas Mulcair is firmly rejecting the Conservatives' choice for the next privacy commissioner, sending a letter to Prime Minister Stephen Harper to lay out his concerns about the appointment.

The government has proposed Daniel Therrien, who is currently the assistant deputy attorney general for public safety, defence and immigration.

In the letter to Harper, obtained by CBC News, Mulcair tells the prime minister that he has serious concerns with the choice.

"I have come to the conclusion that Mr. Therrien has neither the neutrality nor the necessary detachment to hold this position," Mulcair wrote.

The leader of the Official Opposition points to Therrien's involvement in crafting some of the very pieces of legislation he would have to evaluate as privacy commissioner.

For instance, Therrien was involved in negotiating the privacy rules around sharing information for the Canada-U.S. security perimeter deal.

'Imprudent'

Mulcair goes on to tell Harper, "It would be imprudent to place a former civil servant in charge of warning the public about policies he helped design and implement."

The privacy commissioner oversees how government departments and agencies are handling Canadians' private information. The last privacy commissioner and the interim commissioner, who is currently leading the office, have both spoken out in the past about government legislation, along with other privacy issues such as Facebook and Google Streetview.

Earlier this month, interim commissioner Chantal Bernier said the federal government is scooping up data from social media sites and urged the government to sort out its rules for collecting data on Canadians.

The Privacy Act stipulates the appointment must be approved by resolution in the House of Commons and the Senate.

A news release from the Prime Minister's Office says Therrien has been a lawyer at the Department of Justice since being called to the bar in 1981.

In his letter, Mulcair calls on Harper to reconsider Therrien. The NDP, he says, will not support the nomination.