"Flawed" planning and "unrealistic" proposals has left Birmingham City Council facing a budget overspend of £49m, a report says.
In its latest update to the government the Independent Improvement Panel, set up in 2015 to help the council, says its future success "is at risk".
A further £78m of savings are required to balance the 2017-18 budget.
The panel said the council agrees there should be independent review of that budget and its future plans.
The authority faces a "mammoth task" over a short timescale, the panel said in its report to communities and local government minister Sajid Javid.
It acknowledges a "significant number" of the budget reduction proposals recommended in March were unrealistic and many previously undeliverable proposals were carried forward in the 2016-17 budget, the panel said.
The panel was set up after a review by Sir Bob Kerslake in 2014 suggested an independent body was needed to oversee changes at the authority.
Despite making improvements, the panel said progress had been too slow and it planned to give continued support to the council, particularly the senior team.
Council leader John Clancy said he welcomed the panel's acknowledgment "just two years into a five-year improvement programme" that it "has already made considerable progress".
He said: "The panel's acceptance that the council is actively addressing concerns raised by the Kerslake Review and is strengthening how it manages its corporate performance by putting in place improved collaborative working between elected members and officers is also welcomed."
But he said it was correct to recognise the extent of the "extremely challenging circumstances" facing the council, not helped by year-on-year cuts from central government.
Chief executive Mark Rogers said the authority would not "shy away" from the challenges ahead as his team would "not lose sight of our responsibilities".