Disgraced former Co-op Bank chairman Paul Flowers was the only former senior executive of the member-owned company who refused to co-operate with a probe which has led to a damning report on the running of the business.

Report author Sir Christopher Kelly said Mr Flowers, a Methodist minister and former Bradford councillor, who last month was charged with possession of drugs, was the only member of the wider leadership of the Co-op group or bank who declined to meet him.

Sir Christopher, a former Treasury mandarin, has laid the blame for the near collapse of the bank on executives running the business in the wake of its ill-fated merger with the Britannia building society.

He said: “This report tells a sorry story of failings in management and governance on many levels.”

In addition to Mr Flowers, the Kelly report is also scathing about other key figures involved in the debacle – Neville Richardson, its former chief executive, and Bradfordian Peter Marks, former chief executive of the wider group.

His report found “overwhelming” evidence that Britannia chief executive Neville Richardson, who took over as boss following the 2009 merger, failed to leave the new business “in a good position” when he departed in 2011.

The board of the Co-operative Group, led at the time by Mr Marks, was also criticised for failing properly to oversee the bank and badly letting down the group’s millions of members.

Sir Christopher said: “The roots of the shortfall lie in a merger between the bank and the Britannia building society which should probably never have happened.

“Both organisations had problems. Bringing them together exacerbated those problems. It might have worked if the merged organisation had first- class leadership. Sadly it did not.”

The report also focused on a disastrous attempt to update the banking group’s IT systems and the abortive attempt to buy more than 600 branches from Lloyds Banking Group.

Sir Christopher said: “The Co-operative Bank executive management failed in its oversight of the executive. The group board failed in its duty as a shareholder to provide effective stewardship of an important member asset. Collectively they badly let down the group’s members.

“I hope my report will help the group and the bank in their efforts to rebuild organisations of which their members and customers can once again be proud.”

The Co-operative Group welcomed the report, saying it demonstrated the need for change in its own governance. It recently announced record losses of £2.5 billion, largely caused by the bank’s near-collapse.

Interim chief executive Richard Pennycook, a former Morrisons finance chief, said: “It is a sobering assessment which shows clearly that the Co-operative Group’s loss of control of its bank could have been avoided.”

Mr Marks could not be reached for comment.