A mental health hospital ward for children and young people has been rated inadequate after concerns were raised to the Care Quality Commission (CQC).
Priory Hospital Ticehurst House in East Sussex was previously rated "good".
Since then, though, staff numbers have fallen and the CQC found it was no longer safe or well-led.
The number of occasions when children were physically restrained had also increased, the CQC said.
The inspection was carried out after concerns were raised by families and the public, highlighting poor staffing levels, high use of agency staff and poor medicines management.
Kevin Cleary, CQC's deputy chief inspector of hospitals, said the level of care "fell well below the standard that children and young people should expect".
A warning notice was issued ordering immediate improvements. Inspectors returned for an unannounced inspection on 12 December, and found some progress.
Mr Clearly added: "It will take time for the improved processes to demonstrate the sustained improvements but we will continue to monitor the hospital closely."
A hospital spokesman said it was "clearly disappointed" with the rating, and despite significant investment was still experiencing "considerable staffing challenges" on the wards due to national shortages.
He added: "However, since the inspection in September, we have taken immediate and significant steps to address the concerns raised, and the very positive re-inspection feedback from the CQC a week ago shows we are now working in a way that demonstrates improvements in all areas including in relation to incident management, governance and clinical leadership.
"Ensuring young patients with complex conditions receive the best treatment is our overriding priority, and we remain confident of restoring the CAMHS (child and adolescent mental health services) to its previous 'good' rating in line with the overall rating for the rest of the hospital."