Prime Minister Gordon Brown has launched a staunch defence of national tests just days before teaching unions step up their campaign to end key stage 2 assessments.
Writing exclusively in today's TES, Mr Brown says primary school Sats are as important as GCSEs or A-levels in holding schools to account.
Mr Brown writes that the Government is now in a position to devolve more power to heads and give teachers more flexibility - but only if schools can be held properly accountable.
"I'm not willing to accept excuses for underperformance," he said. "Every school should be doing the best by every one of its pupils.
"But progress also relies on the need to retain clear accountability through testing. This means at the end of primary school just as much as at the end of secondary."
Mr Brown's comments come as the NUT and NAHT prepare to launch the next phase in their campaign against key stage 2 Sats.
The unions are calling for next year's tests to be boycotted unless the Government delivers fundamental changes to the testing regime.
They want tests to be scrapped and replaced by teacher assessment, arguing that the existing system forces schools to teach to the test and focus on a narrow curriculum.
On Monday, the unions will announce the results of a petition against the tests, which is believed to have generated more than 20,000 signatures. Authors, including Philip Pullman and former children's laureate Michael Rosen, are among supporters.
In what will be make or break for the unions, they will also begin research to gauge the strength of feeling against Sats among their members.
The NUT will hold an indicative ballot on the boycott during November before deciding on whether to go ahead with a full-scale ballot next year. The NAHT will hold a consultation of its members run by an external company.
Christine Blower, general secretary of the NUT, said: "It is important that we continue to make the educational and professional arguments.
"We have been opposed to the tests since their inception. With the NAHT, we represent the vast majority of heads in primary schools so this is the year where we can absolutely make the case."
Ms Blower and Mick Brookes, NAHT general secretary, are holding on-going discussions with the Department for Children, Schools and Families about the tests. These include the role of the new report cards that could replace highly contentious school league tables.
"At the moment we are saying we have positive and constructive alternatives," said Ms Blower.
"I cannot say the Government looks like pulling away from Sats, but that won't stop us talking to them and trying to resolve this."
The timing of any boycott will be a particular concern for the Government as it will take place in May, widely tipped to be the date for the next general election.
Ministers will not want to be dealing with an industrial dispute at that time, but neither will they want to be seen as caving in to union demands.
Mr Brown said that it had been necessary to have a "rigorous regime of targets and top down programmes" to drive up standards. But he added that having achieved better standards it is now time to show more trust in teachers.
"Trusting teachers is why we have thinned out the curriculum in secondary schools and plan the same in primaries, ensuring a continued focus on the basics alongside flexibility for teachers to ensure that every pupil is stretched and engaged in the classroom," he said.
"Trusting teachers is why we are winding down the national strategies and devolving power to schools to decide themselves how they spend the money for raising standards."
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