A spokesman said Tubb's departure was a "coincidence of timing" with the resolution over the Virgin Energy misselling scandal.
Tubb has been replaced internally by Peter Hofman, previously managing director for group subsidiary Seeboard.
LE Group said: "Following the reorganisation of LE Group and the appointment of Peter Hofman as managing director, retail and marketing, Gary Tubb has decided to resign to pursue alternative interests."
LE Group encompasses London Electricity, Seeboard and SWEB. The company has a 75% stake in Virgin Energy, its joint venture with the Virgin Group.
LE Group's managing director, Jon Kinsey, left the company last month.