A leaked government document appears to show Exeter's application to become a so-called "super-council" should not have succeeded.
Exeter has to provide more financial details on the change
The city was granted unitary status in July, meaning it can take over services like education, social services and highways.
But Devon County Council claims the document shows the city did not attract enough local support for such a move.
The county is challenging the legality of Exeter's unitary status.
The document, from the Department of Communities and Local Government (DCLG), concerns support from local stakeholders, bodies like the police, business and voluntary organisations.
It suggests that 83.5% of stakeholders were against the bid, with only 14.1% in favour.
The county council claims that means Exeter failed to meet the criterion for getting a broad measure of support which forms sufficient reason for refusing the bid.
Its concerns are published in a detailed breakdown of its legal challenge to the government's decision over Exeter's new-found status.
If a judge grants a judicial review, the DCLG will have to convince the court that its decision was reasonable.
Devon's legal challenge will cost about £20,000, which Exeter councillors have claimed is a waste of taxpayers' money.
The application for a judicial review will be made in two weeks' time.