The number of possible illegal schools identified by Ofsted has risen to 420, up from 359 in March.
However, only 55 schools have been closed or ceased to operate illegally since the watchdog established a special team to deal with the issue in 2016.
Of the 55 settings that have closed or ceased operating illegally, 10 have closed and 45 are complying with the current legislation, Ofsted told Schools Week.
Under current government guidance, if five or more pupils attend an institution for 18 hours a week, it must be registered as a “school” and subject to inspection.
The latest data shows that by the end of July, Ofsted had conducted 274 inspections of suspected unregistered schools, up from 192 in March, and issued 63 warning notices, up from 50 earlier in the year.
Warning notices are issued at the end of an inspection if the inspector believes the setting is operating illegally as a school.
The number of schools to have closed or changed their ways has rose from 38 to 55 between March and July.
The figures are from the latest management information published by Ofsted’s £1 million-a-year illegal schools taskforce. The team was set up to clamp down on settings that educate children without being properly registered.