Springfield House Medical Centre in Oldham was given the lowest possible grade when assessed by the Care Quality Commission for safety, effectiveness, and quality of leadership.
The CQC said the Huddersfield Road surgery also "requires improvement" when it comes to the care it provides patients and its "responsiveness".
The practice has yet to comment.
Inspectors in September found staff training "was not well-monitored".
A previous inspection in 2015 rated the practice as "requiring improvement".
Alison Holbourn, deputy chief inspector of general practice at the CQC, said patients "aren't getting the high quality care which everyone should expect".
"It was worrying to see that training at the practice wasn't well-monitored and there was no evidence of all staff completing the appropriate training to carry out their role.
"We found that the practice weren't responsive to the needs of the people using the service, for example we saw no extended opening times, or programmes for specific groups such as carers."
The CQC said the surgery would be kept under review and could face closure unless sufficient improvements are made during the next six months.
A general practice surgery has been put in special measures after being rated "inadequate" by health inspectors.