The Care Quality Commission immediately moved all 48 residents out of Bentley Care Home in Liverpool.
Inspectors also found a disconnected call bell and dead flies and mouse droppings in the kitchen.
The home, which was run by Prasur Investments Limited, has since closed.
Debbie Westhead, CQC's deputy chief inspector for adult social care, described the home as "appalling".
It was officially rated "inadequate" in all areas, with residents' "lives, health and wellbeing" at significant risk.
She said: "We were left with no other option than to urgently remove their registration and work with Liverpool City Council to immediately relocate residents.
"I have no doubt that if Bentley Care Home stayed open, it would have been a case of when, not if, something awful and unavoidable would have happened."
During the visits in April and May, the CQC found medicines not being safely managed, night staff sleeping on duty and people smoking in the building.
A member of staff told inspectors that a call bell had been deliberately cut off to stop a resident ringing for help as it irritated the staff.
Environmental Health also closed its kitchen because of a serious rodent problem. Inspectors found dead flies in a potato peeler and mouse droppings on the floor.
The official report said: "There were no systems or processes in the home to ensure that the service provided was safe, effective, caring, responsive or well led."
The Bentley Road home had been placed in special measures in 2015 under its previous operator.
A care home had its registration revoked and residents relocated after inspectors found rodents, faulty electrics and staff sleeping on the job.