The NUT teaching union has called for the education secretary Nicky Morgan to step down after almost half of Year 6 children failed to meet the expected standard in reading, writing and maths.
Nationally, 53 per cent of children reached the expected standard in this year’s Sats – compared with 80 per cent of pupils who achieve the expected level 4 in all three last year.
Kevin Courtney, acting general secretary of the NUT, said: "This is extraordinarily demoralising for children who are just about to make the big step up to secondary school. They have been told they haven't reached the expected standard to start secondary school.
"It is really important that we reassure parents and children that this is not an accurate judgement of their abilities. This is not their failure – it's Nicky Morgan's failure."
This week, Ms Morgan told a gathering of headteachers that she saw the results as "a reflection of how well children this year have performed against a new curriculum".
She added: "I believe this is a good start that vindicates our decision to raise standards and will help ensure those who need extra help get the support they need to lay the foundations for a bright future."
But the NUT has today called on Ms Morgan to resign from office following "major failings" around the primary assessment reforms.
Mr Courtney added: "In taking this step and accepting the blame for this disaster Nicky Morgan can simultaneously reassure children that these results are indeed not their failure."
But earlier in the week, Ms Morgan stressed that this year's key stage 2 results should not be compared with other years as this is the first year of the new tests.
In a joint statement, the NAHT headteachers' union and the National Governors’ Association have also urged schools not to draw conclusions from the Sats data as "it provides no intelligence on the rate of improvement of teaching and learning".
Both organisations recognise that many will be "feeling demoralised" and the Sats results will be a "challenge" for schools to deal with.
"Pupils, teachers and parents and all involved in schools should be proud of the work they have put in to implement [the new curriculum and the testing regime] in what has been a very short timescale," they added.
This is not the first time this week that the NUT has clashed with the minister. On Tuesday – the day schools received their Sats results – the union held a strike against funding cuts, academisation and deregulation of pay.
A source close to Ms Morgan said: "Following derisory support for their strike last week this is just another attention seeking tactic from the NUT leadership as their general secretary election draws near. Because the truth is the only people who have labelled children failures following last week's key stage 2 results are the NUT themselves.
"In contrast this government introduced a more rigorous new curriculum because we know that our children are capable of reaching the same high standards as their counterparts across the world. That's why we make no apology for raising the bar so that we can be sure children are able to read, write and add up well by the time they finish primary school. That is what parents expect of us and to do anything else would let children down.
"Rather than constantly undermining achievements of their members the NUT should be celebrating them and making plain the fantastic achievements of teachers in transforming the life chances of the children in their care."