The director of international aid agency Care in Sudan, Paul Barker, says he has been expelled from the country.
Care helps those who have fled violence in Darfur
Mr Barker says he was not given a specific reason, but was told the decision had been taken at the highest level of national security.
He said his agency had faced problems with the government since an internal e-mail about the future of Darfur was leaked to the Sudanese media.
A string of international workers have been forced to leave Sudan recently.
Last week, the Canadian and European Union envoys were told to leave after being accused of meddling in Sudanese affairs.
Although, the EU diplomat was allowed to stay for three weeks to finish his term.
Last year, Sudan expelled the head of the United Nations mission, Jan Pronk.
The Sudanese government gave Mr Barker 72 hours to leave on Saturday. He said he was disappointed and surprised at the move.
Mr Barker said the government believed the leaked email contained a political analysis that was inappropriate for an NGO.
"The email I wrote almost a year ago in October [was] talking about the different scenarios that could unfold in Darfur and what the implications would be for our staff. "To me that's a responsible thing to do in terms of ensuring the ongoing safety and well-being of our staff. The people in HAC [Humanitarian Aid Commission] chose to interpret that as political reporting and inappropriate for NGOs."
At least 200,000 people are believed to have died and more than two million displaced in Darfur since fighting broke out in 2003 between rebels and pro-government militias.
Sudan's government accuses the west of exaggerating the crisis for political ; reasons and says just 9,000 have died.
A 26,000-strong UN and African Union peacekeeping force is to be deployed to the region by the end of the year to help the 7,000-strong AU force that has failed to stem the violence in the region.