France will wipe out a third of the debt owed to it by the world's poorest nations and has proposed that the world's six other major industrial powers do the same, an aide to President Francois Mitterrand said. Mitterrand set out his proposal in a letter sent to the six other leaders with whom he will be meeting at the Toronto economic summit June 19-21, special counselor Jacques Attali said. "It has never been so urgent and so necessary to act to help the countries of the Third World," Mitterrand wrote.