The Care Quality Commission says the Royal Cornwall Hospital at Treliske 'needs improvement'
It follows an inspection of three Cornish hospitals in January.
In a report published today the CQC said incomplete paperwork on some wards at the Royal Cornwall Hospital meant patients were being "placed at risk of not receiving the care they needed."
It also found too many operations were being cancelled or delayed due to a shortage of beds.
St Michael's Hospital in Hayle and West Cornwall Hospital in Penzance were rated as 'good'.
All three hospitals were praised as being well led with caring and hard working staff.