OTTAWA (Reuters) - Canadian Aboriginal Affairs Minister John Duncan resigned unexpectedly on Friday after admitting he had inappropriately interceded with tax authorities on behalf of a constituent.

Duncan leaves a month after thousands of unhappy natives mounted protests across Canada about poor living conditions.

Duncan is the first minister to resign on a point of accountability since the federal Conservatives came to power in early 2006 vowing to clean up government.

He said he had written a character reference letter to the Tax Court of Canada in June on behalf of a constituent who was dealing with the Canada Revenue Agency.

"While the letter was written with honorable intentions, I realize that it was not appropriate for me, as a Minister of the Crown, to write to the Tax Court. I have therefore offered my resignation," Duncan said in a statement.

Heritage Minister James Moore will take over Duncan's duties until a new minister can be named. Duncan, who had heart valve replacement surgery in December 2010, will continue to serve as a legislator representing a constituency in British Columbia.

Last month Canadian Finance Minister Jim Flaherty apologized for breaking ethics rules when he lobbied on behalf of a constituent who wanted to set up a radio station.

(Reporting by David Ljunggren; Editing by David Brunnstrom)