Fewer primary school pupils are reaching the level expected of them in English, national curriculum test results have shown.
The proportion of 11-year-olds reaching Level 4 in English fell by 1% this year, according to figures published by the Department for Children, Schools and Families.
The proportion of children reaching the required standard in maths and science has also stalled.
The results showed 80% of 11-year-olds reached Level 4 in English, down from 81% last year; 79% reached this level in maths, the same as last year and 88% reached this level in science, the same as last year.
The test results were returned to schools on time at the beginning of last month.
Teaching unions have warned that up to one in five of the results may be inaccurate.
Mick Brookes, general secretary of the National Association of Head Teachers (NAHT), raised concerns that the quality of marking has suffered due to the pressure on new contractor Edexcel to deliver results to schools on time.
The exam board took over the tests after former contractor ETS Europe was sacked following last year's marking fiasco.
He told the BBC Radio 4 Today programme that Edexcel had done an "amazingly efficient job under immense political pressure after last year" to get the tests marked.
"However, we are concerned that this may well have been at the expense - particularly in writing - at the quality of marking," Mr Brookes said. "And we have had a number of concerns from members coming in, representing thousands of children, where the quality of marking - particularly in writing - has been erratic, has been harsh, and sometimes just clerical errors where there are two writing tasks and the markers have failed to add up the two scores together."
