The Co-operative Group says its chairman has resigned over a scandal that's erupted following a video that allegedly shows the former chair of its banking unit buying illegal drugs.

The group, Britain's largest mutual society, says Len Wardle resigned because of his involvement in the appointment of Paul Flowers, who was caught in a Mail on Sunday video allegedly buying crystal meth, cocaine and ketamine.

Len Wardle said Tuesday the revelations have raised a number of serious questions for both the bank and the overall group. He says that since he led the board that appointed Flowers, he felt it right to leave ahead of his planned retirement in May.

Co-op has launched an investigation and pledged to examine its governance procedures amid the scandal.