Private schools could face unannounced inspections in future under a shake-up of the system, meaning they will be on a par with the rest of the country's institutions, Ofsted revealed on Tuesday.
The move would affect around 1,000 private schools in England. The rest are inspected by independent inspection bodies.
Under the proposed plans, fee-paying schools will also soon be rated on their pupils' attainment.
The announcement comes amid concerns the quality of teaching in non-association independent schools (those inspected by Ofsted) is often "competent but seldom inspiring".
The changes, included in a consultation document published by Ofsted, will see schools rated on how good they are overall, pupils' achievement, behaviour, quality of teaching and the curriculum and leadership as well as on pupils' spiritual, moral, social and cultural development, and health and safety.
At the moment, independent schools are rated on how well their pupils progress, but in future, pupils' attainment will also be considered, the watchdog said.
Inspectors will judge achievement based on their observations of pupils' work and progress.
Under the current system, private schools are given two days' notice of an inspection, but Ofsted is proposing to scrap this so that inspectors turn up unannounced.
As part of an interview with Huffington Post UK, Labour's shadow education secretary Stephen Twigg, said there was "sense in it".
"Schools can be utterly distracted by the knowledge Ofsted is coming in. I don't think Ofsted are getting an accurate assessment of what's going on if they tell as school in advance so turning up with little notice makes sense," he added.
Pilot inspections of the new system are due to start this September, Ofsted said.
Jean Humphrys, Ofsted director of education and care, said: "The quality of teaching is the key driver of school improvement.
"One of the main findings from Ofsted inspection in this sector is that the quality of teaching in non-association independent schools tends to be competent but seldom inspiring.
"It is vital that our inspection is incisive and rigorous, and that judgments are fair, clear and helpful to a school's further development.
"With these new arrangements we will focus more sharply on what makes teaching truly effective."
Ofsted inspects all non-association independent schools in England, which account for around half of more than 2,000 private schools.