Ofsted chief inspector Amanda Spielman has said that scrapping key stage 3 tests worked against many young people's interests and was a "real loss for many schools".
Formal KS3 tests were run until 2008, when they were scrapped by then schools secretary Ed Balls, who said they would be replaced by regular teacher assessments in the early years of secondary school.
Today, Ms Spielman told the Commons Education Select Committee that this resulted in a rapid loss of direction about what KS3 was for and what young people should be able to do by the end of it.
However, she stopped short of backing calls for a new qualification at the end of the key stage, which was suggested by MP and former teacher Jonathan Gullis.
Ms Spielman warned that creating a new qualification would involve building a new infrastructure and potentially take out a lot of teaching time.
Ofsted: We need more clarity about key stage 3, says Amanda Spielman
She said: "I was working at the centre of an academy chain when the key stage 3 tests were taken away. And I think we saw, particularly in the most disadvantaged schools, pretty rapidly a sort of loss of direction about what key stage 3 was for and what young people should know and be able to do by the end of it.
"And I think the loss of that shape and structure really actually works against many young people's interests."
She also addressed concerns raised by Mr Gullis about the importance of KS3 diminishing.
Ms Spielman added: "I, too, have been concerned about the trend to shortening key stage 3, and extending KS4.
"And, of course, the confounding factor is that there are cases in which that model is used in an intelligent way to make a coherent five-year programme, and where there is no loss of breadth of education, and no loss of flexibility of young people to choose different directions from that.
"But in some cases, and we reported in the first six months of EIF [the Education Inspection Framework] on a number where that shortening and limitation of young people's choices really seemed to be to be narrowing their opportunities, channelling them into premature decisions and often channelling them into early decisions that were going to restrict their post-16 choices.
"So, in short, I think more structure and clarity about the end of key stage 3 in its own right – about what the experience and outcomes that young people should have – would be a good thing."
The decision of some schools to finish key stage three before year nine and run a three-year GCSE programme has been a source of major controversy since Ofsted launched its new curriculum focused inspection framework in 2019.