Ofsted has found pupils being taught in "shocking and appalling" conditions at unregistered and illegal schools with open sewers, rat traps, exposed electrics and temporary buildings stacked on top of each other.
The inspectorate fears that at least 6,000 children are being sent to unregistered schools it has found across the country with the head of an Ofsted taskforce warning this could be the “tip of the iceberg.”
Deputy director Victor Shafiee said there was no oversight of a situation where “the least capable people were left with the most vulnerable children.”
Quick read: New register for home educated pupils
Background: First prosecution for an illegal school
He warned that in some cases, inspectors found teenagers just playing computer games and said that some children were being sent to unregistered alternative providers by local councils.
Ofsted launched a crackdown on unregistered and illegal schools in 2016 and has so far investigated around 500 cases inspecting more than 250 sites and issued 71 warning notices to providers it believed were operating illegally.
Earlier this year, it helped secure the first prosecution for operators of an illegal school and it revealed today that two more court cases would be heard this year and there were several more cases in the pipeline.
Sue Will, senior officer and inspector on the unregistered schools taskforce, said: "I have been really shocked at the health and safety issues that we have seen – I mean quite appalling.
"We are not just talking about rundown places that could do with a lick of paint, we are talking about some not very nice places at all.
"Open sewers, rat traps in rooms, I have seen Portacabins balanced on Portacabins to maximise space, exposed electrical-ware, I've seen holes in walls and floors, I have seen locked fire doors, I have seen holes where children have probably punched plaster walls.
"That is the bit for me that I always find so shocking. When you think that every child in this country is entitled to a free school place, and sometimes I come away from these places and think: 'Why, why would you send your child here?”
Ofsted released figures today showing that since 2016, it has investigated 521 settings, and inspected 259 in England. It estimates that up to 6,000 children are being taught at the sites it has visited to date.
Almost a quarter (23 per cent) of the settings investigated are in London, with the rest spread fairly evenly across the country.
Ofsted said alternative provision is the most common type of setting (28 per cent). Around a quarter (26 per cent) of the settings are general education providers, and a fifth (21 per cent) are places of religious instruction.
Ms Shafiee, said: It just isn't good enough that vulnerable children - a lot of these children are - isolated children, end up in these places where there is no oversight, no quality assurance.
"In truth, unregistered schools are a bad thing. They are bad for the children who go to these settings because they are robbed of their life chances.
"They are bad for the parents, because in some cases the parents are misled. They are misled in thinking this is a school and it is operated. And they are bad for society because we are storing up problems."
"People hang on to the bottoms of lorries, and jump trains to get to this country because education is free.
"So why would you then send your child to an unregistered school and pay for them to go to these settings? It is something that I find unfathomable."
A Department for Education spokesman said: "Today's data shows why our new register of children not in school is so important.
"Illegal schools are unregulated and present a danger to both the quality of education and the welfare of those children who attend them - a register will vastly improve councils' capacity to identify those children and intervene.
"We have already established a joint team with Ofsted and provided them with £3 million to investigate these settings, and continue to work with them and the Crown Prosecution Service to make sure illegal activity is uncovered and justice is delivered."