The chief executive of South Africa's state power utility said he would step down in March, departing the company as Africa's largest economy struggles to avoid blackouts that would hit industries and homeowners.
Brian Dames told a news conference he was leaving Eskom for personal reasons and had not come under any pressure to resign.
But the surprise announcement comes just weeks after the state-owned utility declared a power emergency for the first time in five years, due to tight electricity supply.
That raised fears Eskom was coming dangerously close to bringing back the rolling blackouts that paralysed the economy for weeks in 2008, forcing the closure of mines and factories for days and costing billions of dollars of lost output.
More power blackouts could also hurt the ruling African National Congress in an election scheduled for next year, if voters believe the government has not invested enough in essential infrastructure since the end of apartheid in 1994.
The chairman of Eskom told reporters a search was already underway for a successor for Dames, who became chief executive in 2010.
The rand currency weakened after the announcement, hitting a 4-1/2 year low, although traders said the softening was due to technical factors and not directly related to Dames' resignation.
Eskom Chairman Zola Tsotsi told a media briefing that Dames was stepping down for "personal reasons" and that a search for his successor was already underway.