Thousands of the brightest secondary school pupils are being let down by a chronic lack of ambition among some teachers who are failing to stimulate their students' progress, the chief inspector of schools has said.
Children from poor and disadvantaged backgrounds are particularly susceptible to underachievement because teachers are focusing on getting GCSE pupils on the crucial C/D borderline into the "top-grade" bracket, rather than supporting the most able to secure the top A/A* grades, Sir Michael Wilshaw said.
In a nationwide assessment, the head of the schools watchdog Ofsted described the "bleak picture of under-achievement and unfulfilled potential" among secondary school pupils.
He said: "As chief inspector, I have consistently lamented the failure of too many secondary schools to stretch our most able children, particularly the poorest.
"If our nation is serious about improving social mobility then our secondary schools have got to start delivering for these children."
Sir Michael, who is due to stand down in December, said there was a "depressing" trend of the brightest children from disadvantaged backgrounds being the most likely not to achieve their full potential, based on their academic progress between primary and secondary school levels.
The investigation found that of the most able children from poorer backgrounds attending a non-selective secondary school, 64% achieved a top grade (B or above) in GCSE maths, compared with 81% of others.
Some 66% of those from a disadvantaged background achieved a top grade in GCSE English - well behind the 79% of those who were not disadvantaged to achieved a B or higher.
Sir Michael said: "Our nation's economic prosperity depends on harnessing the talent of all our young people but especially those who have the potential to be the next generation of business leaders, wealth generators and job creators.
"As a nation, we have a problem with low productivity. The fact that so many of our poorer bright children are being deprived of the opportunity to fulfil their early promise must surely be one of the underlying causes of this."
Sir Michael said rigorous testing in primary schools was key to helping tackle the attainment gap between poorer pupils and their peers, something he described as a "national scandal".
Calling on the Government to consider reintroducing external national testing for 13 and 14-year-olds, he said: "I firmly believe it was a mistake to abolish these tests in the first place.
"If we are serious about helping all disadvantaged children, but especially the most able, to learn well and unlock their full potential, we need to know how they are doing at 14 as well as at seven, 11 and 16."
He suggested schools should face "further sanctions" if they "consistently fail their brightest pupils". Currently, schools can be placed under special measures if inspection reports identify areas of major concern.
Sir Michael said: "This might seem draconian but unless we get this right as a nation, we will not only continue to let down thousands of our most able pupils but also thwart any ambition to match the productivity levels of our international competitors."
In a statement, a Department for Education (DfE) spokesman said the Government's reforms "are raising standards for all children", with 1.4 million more pupils in good or outstanding schools than in 2010.
The DfE added: "And like Sir Michael Wilshaw, we recognise that more needs to be done to ensure the most able children fulfil their vast potential.
"That is why we are introducing new world-class GCSEs that will stretch the brightest pupils. Alongside this, academies and innovative free schools, such as the King's Maths School, which offers young mathematicians and scientists the opportunity to study alongside leading academic experts, are giving every child the opportunity to fulfil their potential.
"However we are not complacent, and that is why our recent White Paper goes even further by committing to investigating and funding approaches to help even more bright children fulfil their vast potential."