A stand alone council for Exeter could cost tax payers £14m, according to a report commissioned by county councils.
Exeter's move is supported by all political parties on the council
Exeter wants to take control of education, social services and trading standards from Devon County Council.
It says that unitary status would avoid the duplication that exists now through sharing power with the county council.
But a Cambridge academic said the city's bid for unitary status by 25 January next year was "seriously flawed" and expensive to set up.
At present Exeter is responsible for a wide range of local services including planning, housing, waste collection, environmental health and leisure services.
Professor Michael Chisholm, Emeritus Professor of Geography at Cambridge University, who wrote the report commissioned by the County Councils Network, said: "The fundamental problem is that Exeter is quite a small city and previous evidence shows there is no increased efficiency, in fact there is increased inefficiency in having a small unitary authority.
"My upper figure was £14m for the cost of change, but I can see no evidence that the cost would be repaid from savings."
'Good for Exeter'
His conclusions are backed by Devon County Council leader, Brian Greenslade, who does not think that Exeter should join Torbay and Plymouth in becoming a unitary authority.
However, city council leader Roy Slack said the current system left Exeter at a disadvantage.
He said: "Devon County Council's expenditure is skewed further away from Exeter than it should be.
"We are the local council and we should be able to direct that money where necessary.
"It would be good for Exeter and good for Devon."
The bid has all party backing on the city council.