The Office for Standards in Education said the pound;2,250-a-year Newborough school in Liverpool failed to meet national registration guidelines. It has been given three months to rectify "serious" safety violations.
In a report, inspectors said the school was too slow to clear staff with the Criminal Records Bureau and only carried out "ad hoc" checks on employees' identity, medical fitness, work history and professional references. Ofsted said no fire risk assessment had been carried out, despite a request from the fire brigade, and extinguishers were left unchecked "for a number of years".
Owners of the co-educational school, which has 29 pupils aged up to 16, were criticised for neglecting the building, giving children easy access to "dangerous chemicals" and failing to secure a cellar door next to the nursery.
In all, Newborough was violating five out of seven key areas of the Independent Schools Standards 2003. The findings follow criticism of private schools by Ofsted earlier this year.
David Bell, the chief inspector, said, on the whole, standards at private schools had improved during the past 12 months but still fell below acceptable levels. In 2003 Ofsted said four out of 10 private schools that were inspected failed to meet statutory requirements. All schools get three months to address safety or curriculum concerns or face closure.
This year the proportion of schools not meeting health and safety standards fell to a quarter. However, the findings did not apply to members of the Independent Schools Council, which accounts for 80 per cent of children in the private sector and has its own inspection body. Of those schools, only one in seven failed to meet all legal requirements, including keeping correct admissions or attendance registers and carrying out staff checks.
Newborough school, which won praise from Ofsted for good teaching and pupil behaviour, declined to comment on the report.
A spokesman for the ISC said: "All ISC schools are required, as a condition of continued membership, to undergo rigorous inspection at least every six years to ensure that standards are maintained and improved. Our inspection is more extensive and searching than that conducted by Ofsted on schools outside ISC; the latter is to determine whether minimum standards have been met."