Gordon Brown's renewed pledge to clamp down on poorly performing schools has been undermined by a parliamentary report which concludes that judging schools on raw exam scores is fundamentally flawed.
The National Audit Office (NAO) report says there is "no quantified evidence" that league tables or similar schemes help to improve performance.
It warns against setting targets for schools based on raw scores, saying it is fairer to judge them on the value they add to pupils' education.
The findings undermine the Government's National Challenge, which targets secondaries falling below the School Secretary's benchmark of 30 per cent of pupils passing five good GCSEs.
Gordon Brown promoted the scheme in his speech to the Labour conference this week, saying he would guarantee parents a "fundamental right" to have their children taught in schools with good results.
"Our pledge is that any parents whose local state school falls below the expected standard will have the right to see that school transformed under wholly new leadership, or closed and new school places provided," he said.
The audit office report was based on research, interviews with officials involved in public sector programmes, and a review of measures designed to raise results, including league tables and placing schools in "special measures".
It suggests that contextual value added results are a better measure because they take account of factors beyond teachers' control.
"`Threshold' schemes, which target absolute levels of performance and do not take past performance into account, may not reward (those) who improve the most as a result of starting from a lower base," the report states.
It also says that, although more than 60 per cent of those using sanctions or rewards felt performance had improved, such judgments were subjective.
Further criticism of the testing system came this week from:
- an Organisation for Economic Co-operation and Development report, which looked at social inequality in UK schools and recommended we "reduce the focus on testing and targets and put more focus on supporting weak students and schools".
- and local authorities, some of which criticised targets for five-year- olds as "nonsense". The difference between local authorities' results and their targets differed wildly this year, partly because the targets were set two years ago.
The Department for Children, Schools and Families welcomed the audit office report as it was "entirely consistent with our approach to improving school performance".
A spokesman said: "The NAO says that performance measures must allow organisations to focus their goal-setting correctly and precisely - and 30 per cent of children leaving with five A*-Cs, including English and maths, is a clear goal."
He added that the DCSF had recently been praised by the OECD for school reforms that exemplified "much of the best current wisdom".