Investigations into the "homes for votes" scandal at Westminster City Council should stop, says district auditor John Magill.
Dame Shirley Porter owes millions of pounds in surcharges
He said it was not in the public interest to spend more money on inquiries.
The council will, however, chase former council leader Dame Shirley Porter for the £37m she owes in surcharges.
The scandal involved council homes being sold to potential Tory voters in marginal wards.
Assets worth millions of pounds said to be controlled by Dame Shirley, who now lives in Israel, were frozen under court orders secured by the council last November.
The council has spent hundreds of thousands of pounds pursuing the money.
Dame Shirley claims the council's investigators had sought to involve off-shore trusts whose principal beneficiaries had no connection to the case.
Mr Magill said on Tuesday his report was intended to draw a line under the scandal.
"In my view it is in the public interest that the council and others should look forward rather than back," he said.
He said only the conduct of Dame Shirley, her former deputy David Weeks and former council managing director Bill Philips could justify further investigation and they had already been criticised by the Divisional Court.
The current leader of Westminster, Simon Milton, welcomed the report.
"As leader of today's council, I apologise for the mistakes of some of my predecessors," he said.
"As a group we are determined those mistakes will never be repeated."
Council chief executive Peter Rogers said: "The city council welcomes the fact that the appointed auditor's report contains no recommendations for the council and draws a line under the past, as no further consideration or action by the auditor is justified.
"We have changed substantially since the late 1980s.
"Our arrangements are robust and our processes are open and transparent."