Len Wardle, the long-standing chairman of the Cooperative Group, has resigned with immediate effect after he admitted "serious questions" were raised by the drugs scandal over former banking chairman Paul Flowers.

Mr Wardle, who has held the position since 2007, announced last month that he was due to leave next May but he said it was now right for him to go straight away, having led the board that appointed Mr Flowers.

Mr Flowers, a Bradford-based Methodist minister who chaired the Co-operative Bank for three years from 2010, was exposed allegedly buying and using illegal drugs in a newspaper sting at the weekend.

Mr Wardle said: "The recent revelations about the behaviour of Paul Flowers, the former chair of the Co-operative Bank, have raised a number of serious questions for both the bank and the group.

"I led the board that appointed Paul Flowers to lead the bank board and under those circumstances I feel that it is right that I step down now, ahead of my planned retirement in May next year.

"I have already made it clear that I believe the time is right for real change in our operations and our governance and the board recently started a detailed review of our democracy.

"I hope that the group now takes the chance to put in place a new democratic structure so we can modernise in the interests of all our members."

Mr Flowers, a former Bradford councillor, now faces a police investigation. He has apologised over the scandal and been suspended by the Methodist Church as well as the Labour Party.

Mr Wardle's departure will see him replaced by his deputy, Ursula Lidbetter, chief executive of the Lincolnshire Co-operative.

The Co-operative Group said: "It is intended that Ursula will chair the group through the current governance review, which will include consideration of how the board is constituted and chaired."