One in 20 children in England is in a school that has not been inspected for more than 10 years, the BBC has found.
Analysis of official data revealed 24 schools had gone without inspection for more than 13 years. There are 1,010 "outstanding" schools that have not had a visit from Ofsted in a decade - up from 296 in 2017.
The Department for Education (DfE) plans to lift an exemption on routine inspections for outstanding schools.
It has been in place since 2012 but watchdog Ofsted called for it to end amid concerns about falling standards.
The National Association of Head Teachers (NAHT) said inspecting all schools would ensure they were not "failing or at risk of decline"
It said parents would find it harder to compare schools without recent reports available.
Meanwhile, the Labour party has said it would scrap Ofsted if it won the next general election.
There were 24 schools that had not had an inspection since September 2006, all of them graded "outstanding".
The latest data showed more than half of previously outstanding schools lost the top grade at their most recent inspection.
There were 56 schools that were previously graded outstanding that dropped to the lowest mark, "inadequate", in Ofsted reports over the past five years. Another 159 dropped to "requires improvement" while 1,203 went from outstanding to "good".
A spokeswoman for Ofsted said the watchdog had been calling for the exemption to be lifted and was pleased by the DfE's announcement.
