The good work done by primary schools to improve children's spelling and grammar is being "lost" when they enter secondary education, causing "an enormous waste of talent", the head of Ofsted has said.
Sir Michael Wilshaw, chief inspector of schools in England, said it was "little short of a tragedy" that thousands of children who achieved high grades at the age of 11 were failing to "fulfil their potential" when they took their GCSEs.
He said he had "great cause for concern" about the transition from primary to secondary education and warned it was "particularly damaging" for the most able pupils from poorer backgrounds.
In the first of a series of monthly commentaries, Sir Michael said: "My inspectors tell me that much of the good, structured work done in primary schools on understanding and using correct grammar, both when writing and when speaking, is lost when pupils enter the secondary phase.
"Worse still, the rigour with which spelling, punctuation and grammar is being taught at primary stage is often not developed sufficiently at secondary stage, especially in the foundation subjects like history and geography.
"This slows down all children, but is particularly damaging for the most able pupils from disadvantaged backgrounds who disproportionately fail to fulfil their earlier potential when they come to sit their GCSE examinations."
Sir Michael said primary schools had successfully countered the "misguided ideologies of the 1970s and 80s" which had resulted in "generations of adults" having never been taught the basics of grammar at school.
But he pointed to figures in 2014 which showed around 5,000 disadvantaged pupils, who attained the highest levels at the end of Key Stage Two, had failed to achieve a grade B in English and mathematics at the age of 16.
"This is little short of a tragedy for the young people concerned and an enormous waste of talent for our country," he said.
Sir Michael said he would offer his "full support" to plans to introduce resit tests for Year Seven pupils who do not achieve the expected standards for 11-year-olds.
He added: "I stand fully behind any action that will sharpen the focus on the vital transition between primary and secondary school."