OTTAWA (REUTERS) - Canadian Aboriginal Affairs Minister John Duncan resigned unexpectedly on Friday after admitting he had inappropriately written a letter to a tax court on behalf of a constituent.

Mr Duncan leaves a month after thousands of unhappy natives mounted protests across Canada about poor living conditions. He is the first minister to resign on a point of accountability since the Conservatives of Prime Minister Stephen Harper came to power in early 2006 vowing to clean up Ottawa.

Finance Minister Jim Flaherty was formally reprimanded last month for urging the telecommunications regulator to grant a radio licence to a company in his parliamentary constituency. He kept his job.

Mr Duncan said he had sent a character reference letter to the court in June 2011 on behalf of a constituent who was dealing with the Canada Revenue Agency.

"While the letter was written with honourable intentions, I realise 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," Mr Duncan said in a statement.

Government ministers are not allowed to lobby regulators or other authorities in their capacity as Cabinet members.

The official opposition New Democrats said the case showed the government had ethics problems and complained that it was inconsistent for Mr Duncan to resign and Mr Flaherty to stay in his job.

"It seems like they're increasingly out of touch with everyday people in terms of how you apply fair rules," senior legislator Paul Dewar told the Canadian Broadcasting Corp.