Almost a third of local authority secondary schools in England are unable to cover their costs, a study suggests.
The Education Policy Institute says its research shows the proportion of such schools with budgets in the red has almost quadrupled in four years.
And the average local authority secondary school debt is £483,000.
But the Department for Education says that across all types of state schools, more than 90% are in surplus.
Former Education Minister David Laws, who chairs the Education Policy Institute (EPI), said the latest school budget figures, for 2017-18, showed a "marked deterioration".