The source said Swiss-based Vitol has made the decision because it had made a loss of an estimated $70 million for the calendar year 2007 from its existing contract. The source did not mention any political considerations.
Vitol’s spokeswoman and Iranian oil officials were not immediately available to comment.
Iran is the world’s fourth-largest crude oil exporter but its refineries are struggling to fully meet domestic demand.
Western sanctions mean it is hard for the country to develop refineries and the need to import the fuel could make it more vulnerable to pressure over its nuclear programme.
Trading sources have said Vitol has been the biggest gasoline supplier to Iran, handling about 60 percent of the country’s petrol imports.
The industry source said it is not clear how or through which intermediary Iran would get alternative supplies to replace those handled by Vitol.