A legal anomaly means that these nurseries - which make up the fastest-growing group in the private sector - fall outside the remit of the Early Years Directorate, the new arm of the Office for Standards in Education.
From September next year, it will inspect 80,000 childminders and 25,000 nurseries, creches and playgroups to ensure they meet national care standards.
Criticism of the omission has come from the National Day Nurseries Association which claims it will undermine Oftot's mission of bringing consistency to the early-years' sector.
The Government, which has the power to remove the exemption under the newly-passed Care Standards Ac, is now in talks with the private sector.
A spokeswoman for the Department for Education and Employment said: "We are considering what registration and inspection arrangements would be appropriate."
Nearly 80,000 two to four-year-olds are taught in private nurseries attached to prep schools. Of the 12,080 schools that are members of the Independent Schools Council, 865 take pupils under the age of five.
At present, private schools are inspected by either HMI or OFSTED. The 60 per cent of schools affiliated to the council are checked by their own inspectorate.
The national care standards, which cover 14 areas, including staffing, health and safety, care and learning and equal opportunities, are out for consultation until October 31.