Almost one million children are being taught in underperforming schools in England, MPs have warned.
The cross-party public accounts committee claims 1,557 schools are underperforming, including four per cent of primary schools and 23 per cent of secondaries.
This is despite government funding of £837 million to improve standards, which is in addition to the £25 billion already invested in schools last year.
Committee chairman Edward Leigh called for tougher action on underperforming schools and in particular for a more rigorous inspections regime for those doing badly.
However, the government has accused the MPs of exaggerating the problem and teaching unions have also condemned the report as "misleading" and an "oversimplification".
The committee finds that in July last year, education watchdog Ofsted had put 242 schools in special measures - the most failing schools - a further 286 had serious weaknesses and another 49 were underachieving.
In addition, the Department for Education and Skills (DfES) had classified 402 schools as low attaining and 578 as underperforming. Although the total is less than previously, these schools involve 988,000 pupils.
"To waste so much human potential in this way is a tragedy. The consequences in the long term for the pupils themselves and, more widely, for our society will be severe," said Mr Leigh, a Conservative MP.
He said the signs of decline needed to be picked up earlier to allow action to be taken, and although he noted information on secondary schools is improving, he warned similar data on primary schools was particularly hard to come by.
Mr Leigh also criticised the new "lighter-touch" Ofsted inspection regime, saying underperforming schools were not getting the support they needed. In particular, he warned that head teachers may be "over-generous" in evaluating their own performance.
"The leaders of weaker schools are hardly the best people to evaluate their own performance. Inspection reports must include a distinct assessment of the head teacher," he said.
However, schools minister Jim Knight said the committee had mixed "apples with pears" to reach the 1,500 figure, mistaking failing schools and those that are simply performing less well than others with similar characteristics.
"This is not the picture of education that parents, pupils and teachers see up and down the country and it is insulting to their hard work," he said, adding that in some of these so-called underperforming schools, 60 to 70 per cent of pupils get five good GCSEs.
Steve Sinnott, general secretary of the National Union of Teachers (NUT), condemned the committee report as a "gross simplification of the reality of education in our schools", saying schools experience difficulties for many reasons, not just because they are failing.
"This is an alarmist scare story but parents should be reassured that their children are receiving high quality in the vast majority of the country's schools," he said.
The Association of School and College Leaders (ASCL) also rejected today's report as "misleading and damaging", and general secretary John Dunford accused Mr Leigh of being "irresponsible".