The Audit Commission's job of assessing councils will become "unworkable" if judges uphold a ruling that it wrongly downgraded Ealing Council because of its zero-star social services department.

That was the regulator's message in an appeal against a High Court decision that it was wrong to rate Ealing as "weak" in the comprehensive performance assessment as an automatic consequence of its zero-star rating.

The High Court had ruled that the commission had effectively ceded its statutory function of determining a council's overall rating to the Commission for Social Care Inspection.

Had it not been applied, Ealing would have been rated "good".

But the commission claimed the court's ruling would make its duty to assess local government performance impossible, saying it was "erroneous in law and unworkable in practice".

The Appeal Court will deliver its verdict at a later date.

• Ealing Council has appointed David Archibald as executive director of education, social services and housing. In his last post as director of social services at East Sussex Council he led the authority from zero stars to two in 2003.