Isle of Anglesey County Council has lost its leader only weeks after the abrupt departure of chief executive Derrick Jones, who served for two and a half years.
Phil Fowlie, who led the ruling Original Independents group, said: “It is with regret that, following medical advice, I have confirmed my resignation as council leader.”
The council is subject to a Wales Audit Office investigation, which is due to report next month, called after its annual audit letter expressed concern about “difficulties in relationships between some executive members and senior officers”.
North Wales Police received an allegation that Mr Jones had suffered harassment but no action has been taken.
Alastair Robertson, secretary of the Association of Local Authority Chief Executives, said at the time that Mr Jones did not believe himself to have been harassed.