"Tens of millions" of pounds of public money has been wasted by London's local government, according to a report.
But the Forensic Audit Panel found former mayor Ken Livingstone and his team did not break any rules, blaming ineptitude rather than corruption.
It recommended the London Development Agency (LDA), the mayor's business arm, should in future only commission, and not deliver, projects.
Mr Livingstone refused to talk to the panel, saying it was not independent.
The London Assembly Labour group also criticised the publication for lacking independence and credibility.
London Mayor Boris Johnson commissioned the report, headed by Patience Wheatcroft, calling for greater transparency within City Hall.
It comes as the Metropolitan Police continue to investigate allegations of fraud in connection with six organisations which received funding from Mr Livingstone's office.
In March Lee Jasper, Mr Livingstone's race advisor, stepped down amid allegations over the misuse of public funds. He has denied any impropriety.
He told BBC News: "I always maintained that nothing untoward happened, that every action we took as mayoral advisers was in the rules of the game.
"They found no evidence of corruption despite a forensic audit and I am happy for that."
At a briefing ahead of the publication of the report on Wednesday, Mrs Wheatcroft said: "We are not giving Mr Jasper's actions a clean bill of health. That's something that the Met (Police) is looking at.
These people had a lot of power and they used it
"Whatever the outcome of these investigations, we are of the view that it was ineptitude rather than corruption that was the biggest blight of the London Development Agency (LDA)."
She said that while Lee Jasper and other mayoral advisors did not appear to break any rules "these people had a lot of power and they used it."
Mrs Wheatcroft called for the LDA to be reshaped and £8m worth of savings to be made at the Greater London Authority (GLA).
The panel looked in detail at eight cases where LDA had given grants to projects.
It concluded that few checks were made to ensure the cash achieved the objectives for which it had been allocated.
She said she was not able to give an exact figure on how much money had been wasted but added: "I think you can say tens of millions have been wasted."
She said the panel had found money was being "mis-spent on a grand scale" at the LDA which worked to serve "as the mayor's chequebook".
Mrs Wheatcroft said "significant cost savings" could be made across the GLA and called for a new panel to assess whether Transport for London was providing value for money.
But Labour Assembly member John Biggs said the report had been "compromised by its complete lack of independence from day one".
"To portray this as some kind of independent report rather than a political hatchet job is to mislead Londoners," he said.
In response to the findings Mr Johnson said: "There clearly needs to be less duplication, more accountability and real improvements made to management within the GLA and LDA.
Speaking on BBC News, Mr Livingstone said there was no identification of the tens of millions of pounds that were said to be wasted in the report and no projects were listed.
"When you actually plough through all the 90 pages, they can't specify any of this.
"There are also different priorities mayors have," he said.
"I made the decision that should we have a tourist attraction in Newham, that can bring a million tourists to this borough. I think that's the right decision, they don't agree with it."
He also said that everything had to be cleared by auditors and treasurers.
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