A bus company is seeking a judicial review over plans to bring Greater Manchester's bus network back under public control.
It comes after a public consultation into the proposal to operate a London-style system where operators bid to run services on a franchise basis.
But Stagecoach has accused the Greater Manchester Combined Authority (GMCA) of conducting "a flawed consultation".
The GMCA said it had filed a response to the judicial review application.
If approved, the city-region would be the first area outside of the capital to have a regulated bus system since the 1980s, the Local Democracy Reporting Service has said.
Stagecoach has now submitted a court application for a judicial review.
Chief executive Martin Griffiths said: "We believe the GMCA conducted an unlawful process and a flawed consultation on proposals which do not properly reflect the fundamental and material changes brought about by the Covid‐19 pandemic."
He added that the proposed scheme involves spending £135m on transitional costs without delivering any improved services for customers.
A decision on the future of the bus network was expected from Greater Manchester mayor Andy Burnham in March 2020 but it was postponed due to the outbreak of Covid-19.
This delay prompted a second consultation where the number of positive responses fell from 83% to 71%.
Mr Griffiths said it was "clear from a significant number of consultation responses that there is widespread concern".
"While we await the decision of the court, we would urge the combined authority to rethink its approach and pause its plans," he added.
A GMCA spokesperson said: "We have reviewed and filed a response to the claim and are awaiting an update from the court on next steps.
"It would not be appropriate to comment further at this stage."