The personnel sub-committee agreed in October to an early retirement deal for city building services business manager Joe Gill, but changed its mind and suspended him late last month pending investigations. He resigned a few days later.
A report from accountant Coopers & Lybrand blamed the unit's management for losses of£3.6m. The district auditor has also started an investigation.
The four assistant business managers at the unit have been made redundant under council restructuring, which will see staff reduced from 216 to 151.
Manual worker redundancies were still being discussed with unions but it was hoped to avoid compulsory redundancies, he said.
The report said there was evidence the unit had bid under cost for construction and highways contracts in an attempt to fend off private sector competition.
It held the management team responsible, saying it failed to provide necessary information, despite there being reporting systems in place to ensure accountability: 'These robust and advanced reporting arrangements placed an obligation on the business manager to provide regular, accurate and comprehensive reports on the performance of the unit.'
The report said tough measures were needed to make the unit viable in a competitive market, although it acknowledged much had been done after intervention from chief executive Colin Sinclair.
Contract services committee chairman Bob Pells said the report, which remains confidential, cleared up uncertainty over how the losses occurred. The council now had to learn from the mistakes, he said.
An Audit Commission spokeswoman confirmed that chief district auditor David Parkin is also investigating city building services. The inquiry was prompted late last year by specific allegations into the unit, she said.
No decision on further action would be taken until the inquiry was complete, and there was no police investigation, she said.
Mr Gill was not available for comment.