Almost half of primary headteachers are worried about the marking of this summer’s key stage 2 national tests, research shared exclusively with TES reveals.

In a poll of more than 1,000 headteachers and senior leaders by the NAHT union, 46 per cent said they “had concerns about the marking of this year’s Sats”. Of these, 78 per cent were worried about the accuracy of marking.

Russell Hobby, general secretary of the NAHT, told TES the survey revealed “basic inconsistencies in marking”. He plans to use the findings to lobby the government for significant changes to the way test results are used.

“In some cases, schools are getting really obvious errors,” Mr Hobby said. “For instance, two children who have given the same answer are getting different marks for it.”

The survey comes in the wake of a series of complaints about the marking of the tests, which was conducted completely online this year for the first time.

The NAHT survey is the first indication of the scale of headteachers’ concerns. More than a third of the respondents said their school’s Sats results this year were not in line with teacher assessments of pupils’ ability.

A spokesperson for Pearson, which runs the tests, said: “We would like to reassure students, teachers and schools that Sats exam papers have been marked at the very highest standard.”