The head of Britain’s biggest independent co-operative group has entered the debate over governance of the group.

Midcounties Co-operative chief executive Ben Reid said it was “tragic” that former City minister Lord Myners had personally vilified the Co-op’s directors for presiding over a failure of governance.

Lord Myners alleged that some directors did not know the difference between a credit and a debit when announcing his report into the crisis at the group on Wednesday. 

He attacked the group’s “labyrinthine” structure and said directors were “in denial” about what they had let happen to the 170-year-old business, which spans grocery stores, funeral parlours, pharmacies and travel agencies.

Reid told the Guardian: “It’s a shame that the debate has got personal when it’s about something more important than Ben Reid or Paul Myners. We’ve got the AGM next week and I look forward to us having a good debate and moving forward.”

Reid and fellow independent co-operatives have a crucial role in determining whether Myners’ reforms are voted through next Saturday because they own 22% of the group, with representatives of the Co-op Group’s 8 million members controlling the rest.

Myners said it was “astonishing” that Reid was still on the group’s board after sitting on the Co-op Bank’s audit committee for three years while a £1.5 billion hole built up in its finances.

Losses at the bank, now only 30% owned by the group, are viewed by Myners as an indictment of how its parent was run, the newspaper reported.

Reid said: “I’m not the only one that has been on the board in that time. There are all sorts of implications of what Paul is saying. Why he would target me I don’t know.”

He has said he is open to change and has denied threatening to block Myners’ proposals in advance of their publication.

“It’s a tragic way to handle what should have been quite a positive thing for the whole of the movement,” Reid said.

“I don’t know why Paul has chosen to make this adversarial whereas the nature of the [co-operative] beast is to want to discuss things, be collaborative and debate.”