Private school inspectors must improve the way they evaluate lessons, a report found yesterday.
The government education watchdog Ofsted said inspectors of independent schools sometimes failed to provide enough evidence to back up the grades they awarded.
Ofsted said the Independent Schools Inspectorate had failed to tackle the problems which were first identified last year.
"The undue variation in the quality of lesson observation forms completed by inspectors remains a weakness," the watchdog said.
"In some cases, for example, limited reference is made to the achievement of pupils of different ability groups.
"In others insufficient evidence is advanced to support the awarded grades."
Ofsted was also critical of the quality of some written reports from ISI.
"Occasionally reports do not have enough information about the provision or outcomes for significant groups of pupils in the school's population, such as those for whom English is not their first language.
"Some do not make sufficient use of evidence in judging the achievement of different groups, such as boys and girls, where this is relevant."
ISI inspects all schools which are members of the Independent Schools Council.
Ofsted inspects state schools and other independent schools as well as monitoring the work of ISI.