Some schools are giving disadvantaged teenagers reading material designed for primary school children, Amanda Spielman has warned.

The Ofsted chief inspector told the Observer she had been driven “absolutely nuts” by schools that assume children from poor backgrounds were “automatically low-achieving and need a watered-down curriculum”.

She said: “One thing that has been pointed out in various contexts … recently is the tendency to use texts, even in secondary education, that only require a reading age of 10 or 11.

“Children can go all the way through secondary school and then go bump when they hit real demands in post-16 education or have aspirations for university, because they just don’t have the experience or practice of reading more demanding texts.”

She added that while schools may think they are helping disadvantaged pupils by providing relevant material, the practice actually “hollows out” education, meaning they “don’t get the experience that they absolutely should”.

The chief inspector repeated warnings about Ofsted’s funding, telling the paper that the inspectorate was at the “very limit of giving the level of assurance that we are expected to give”.

It comes after her comments in May that her organisation was finding it “pretty tough” to do its work on a “shrinking budget”.

On Friday, a heavy-weight commission on how schools are held to account linked cuts to Ofsted’s budget to the questions about the reliability of its reports.

It said: “The choice is stark - either the government chooses to invest heavily to ensure reliable inspection for all, or we revise our expectations about what to reasonably expect from the inspection process.”

Spielman also reiterated concerns about schools off-rolling pupils who could damage their league table position, saying that the schools system had “stopped acting in children’s interests”.