Almost 40% of primary school pupils in England failed to meet the Government's expected standard in reading, writing and maths, national SATs tests results show.
Figures released by the Department for Education showed 61% of pupils reached the expected standard in 2017, up from 53% in 2016.
This means 39% failed to reach the expected standard across all three subjects of reading, writing and maths this year.
More than half a million 11-year-olds across England took national curriculum tests, known as SATs, in May, with the results used in annual school league tables to assess a school's performance.
The figures show an improvement on last year in all areas with 71% of pupils meeting the expected standard in reading, compared with 66% last year, while 75% of pupils met the expected standard in maths, compared with 70% last year.
The expected standard in grammar, punctuation and spelling was met by 77% of pupils, compared with 73% last year, and 76% of pupils met the expected standard in writing, compared with 74% last year.
To reach the expected standard in all of reading, writing and maths, a pupil must achieve a scaled score of 100 or more in reading and mathematics tests and a teacher assessment outcome of "reaching the expected standard" or "working at a greater depth" in writing.
School standards minister Nick Gibb said: "Today's results show sustained progress in reading, writing and maths and are a testament to the hard work of teachers and pupils across England.
"Thanks to their commitment and our new knowledge-rich curriculum, thousands more children will arrive at secondary school having mastered the fundamentals of reading, writing and maths, giving them the best start in life."
The average scaled score for reading in 2017 was 104, up from 103 in 2016.
The average scaled score for grammar, punctuation and spelling was 106 in 2017, up from 104 in 2016.
The average scaled score for maths was 104 in 2017, up from 103 in 2016.
SATs tests have long proved controversial, with teaching unions arguing that they put too much pressure on children, and the results are not a reliable measure of how schools are performing.
Last year, pupils sat tougher papers based on a new national curriculum, and 53% of pupils reached the new expected standard in reading, writing and maths.
The year before, under the old system, 80% achieved Level 4 - the old level expected of the age group - or above in these core subjects. Ministers were at pains to stress that the results were not comparable.
But union leaders branded last year's results a ''shambles''.
Headteachers have warned that primary schools must not be judged on their SATs tests results alone.
The National Association of Head Teachers (NAHT) said schools and parents know that the results must be taken with a "pinch of salt".
Russell Hobby, general secretary of NAHT, said: "Comparisons with last year are inevitable but they are also unwise, as last year's results were unexpectedly low and pupils were being assessed at a time when the curriculum and assessment methods had changed significantly.
"This year, students and schools are more familiar with the new-style tests. Teachers will have had a better idea about what to expect and have been better placed to prepare children."
Mr Hobby pointed out that pupils have also had an extra year following the new curriculum, adding that some of the tests have been designed better this year.
"Still though, it is necessary to take these results with a pinch of salt. And schools should never be judged by data alone.
"NAHT believes that schools should be held to account in a fair way, recognising that test and exam results are only part of the picture when judging a pupil's success or a school's effectiveness.
"Just looking at SATs data misses the majority of the real work that schools do to help young people achieve their full potential.
"Apart from anything else, we need to remember that these results simply reflect how a small proportion of a school's pupils performed in a one-off 45- minute test.
"They are at best a tiny snapshot of a very specific area of a child's development.
"We have written to the Secretary of State this week to push for continued reform of primary assessment.
"We look forward to continuing that dialogue over the summer, so that we can build an assessment system in England that parents, teachers and school leaders can all have faith in," he said.
Dr Mary Bousted, general secretary of the Association of Teachers and Lecturers (ATL), said: "Today's results reflect that children and education professionals have worked extremely hard to get to grips with these new-look SATs.
"But SATs continue to have a negative impact on children's education and the exams are not fit for purpose.
"Preparing for SATs takes up too much class time, with schools focusing on getting children through the tests. It means children spend too much time practising tests and they miss out on other subjects.
"Having a good knowledge and understanding of English and maths is important for all children.
"But the assessment of writing is not fit for purpose and has undermined the teaching of writing in many primary schools, as well as causing significant workload for teachers.
"We call on the Government to urgently reform this aspect of the assessment system.
"The Government must ensure that the data from these tests is not used to unfairly penalise schools and teachers.
"The SATs are too narrow and too high stakes to function as a fair measure for whole school performance.
"SATs are at the centre of a toxic accountability system that is driving teachers and leaders out of the profession."
Meanwhile, tests at a primary school in Orpington, Kent, were annulled following an investigation.
A Department for Education spokeswoman said: "Any instance of maladministration of the National Curriculum tests is completely unacceptable.
"Following an investigation into the administration of 2017 Key Stage 2 tests at Leesons Primary School, all the Key Stage 2 tests - which includes English reading, Grammar, punctuation and spelling, and Mathematics results - were annulled."
The Spring Partnership Trust confirmed an external investigation into the administration of the SATs tests at Leesons Primary School has been carried out by the Standards and Testing Agency (STA) and that, as a result, the STA has had to annul the 2017 Key Stage 2 SAT results for the school.
A statement said: "An internal investigation is being carried out by The Spring Partnership Trust (the Multi-Academy Trust that Leesons Primary School became a part of in December last year) as a matter of urgency.
"We are not in a position to discuss the investigation at this point.
"We can confirm that steps are being taken to ensure that the pupils will not be disadvantaged by the effect of the STA's decision and that during the schools recent Ofsted inspection, Y6 pupil's books were independently scrutinised and judged as being good, and overall the school received a 'Good' Ofsted grade.
"The Spring Partnership Trust is determined to take the right steps so that Leesons Primary School continues to be a school of which the parents, pupils and staff can all be proud."
Commenting on the figures published on Tuesday, Chris Keates, general secretary of the NASUWT, said: "These results reflect the considerable hard work which has been put in by pupils and teachers over the past year, and we should all congratulate them for their achievements.
"That this has been achieved despite the confusion created by the chaotic introduction of the new assessment framework, which barely a year after introduction is already under review by Government, is of great credit to the resilience of the teaching profession.
"Their hard work, skill and dedication while battling the burden of excessive workload, year-on-year cuts to pay, constantly changing policies and ongoing budget cuts remains remarkable.
"For the good of all those involved, the NASUWT reiterates its call for government to conclude its ongoing review of all of the issues surrounding the framework assessment as soon as possible, so we can urgently move to a sustainable system of assessment that is fit for purpose and commands the confidence of teachers and the public."