Aboriginal Affairs Minister John Duncan has resigned after improperly advocating to a tax court on behalf of a constituent.

A rough week for the governing Conservatives got rougher Friday with the surprise announcement, first from the Prime Minister's Office and then from Duncan himself.

Duncan's cabinet duties will be taken up temporarily by Heritage Minister James Moore.

Cabinet members were recently asked to review their correspondence following revelations by The Canadian Press that Finance Minister Jim Flaherty had improperly promoted a business in his riding in its license application to the Canadian Radio-television and Telecommunications Commission.

The federal ethics commissioner reiterated that such interventions by public office holders are forbidden.

Duncan said in a statement that a subsequent search by his office turned up his own improper advocacy.

"In June of 2011 I wrote a character reference letter to the Tax Court of Canada on behalf of an individual to whom my constituency staff was providing case work assistance on a Canada Revenue Agency matter," Duncan said in the statement.

"While the letter was written with honourable intentions, I realize that it was not appropriate for me, as a minister of the Crown, to write to the Tax Court."

Duncan said he offered his resignation to the prime minister and it was accepted.

"I take full responsibility for my actions and the consequences they have brought," said Duncan.

Genevieve Salvas, the acting executive legal counsel for the Tax Court of Canada, said her office was just learning of the matter and had no immediate comment.

"I would like to thank Mr. Duncan for his many contributions as minister and for his service to the people of Canada," Harper said in a release.

"Mr. Duncan will continue to serve as the member of Parliament for Vancouver Island North in the House of Commons."

The week began with Sen. Patrick Brazeau being expelled from the Conservative party caucus as he was being charged with assault and sexual assault in an apparent domestic altercation at his Gatineau, Que., home.

Brazeau was already embroiled in a controversy over his Senate expense claims -- a battle that has now expanded to include at least two other Conservative senators, Mike Duffy and Pamela Wallin. The upper chamber is conducting audits on all three senators, as well as on Liberal Senator Mac Harb.

Duncan is the seventh cabinet minister to resign from the Conservative front bench since Harper came to office in January 2006, but only the third whose departure is linked explicitly to improper behaviour.

Maxime Bernier resigned in 2008 after leaving a secret briefing book at his then-girlfriend's apartment and Helena Guergis stepped down in 2010 amid allegations of criminal activity involving her husband's business -- although no charges were ever laid.

Duncan, 64, has suffered from poor health and was seen as one of the Conservative government's less-accomplished advocates during the last several months of First Nations' protests.

With the prime minister having publicly committed to a new round of negotiations over treaty issues and land-claims settlements, the leadership of the Department of Aboriginal Affairs and Northern Development is likely to take on a much higher profile in the Conservative government.

Section nine of the Conflict of Interest Act prohibits ministers from using their position to try to influence decision-making when doing so would improperly advance another person's private interests.

Guidelines issued by the Prime Minister's Office also say cabinet members should not intervene in courts or license decisions of independent tribunals.

There are precedents for Duncan's resignation.

Jean Charest stepped down as minister of state for youth, and minister of state for sport and fitness, in 1990, after calling a Quebec judge about a case involving the Canadian Track and Field Association.

Bruce Cheadle reports for the Canadian Press. With files from Jennifer Ditchburn.