Headteachers are warning of "serious alarms" over the marking of national curriculum tests for 11-year-olds taken by 600,000 pupils this summer.
The National Association of Head Teachers said many of its members had claimed that pupils had been graded incorrectly – particularly in the writing test. One head reported a number of crossings out and changing of marks by an examiner on a pupil's paper.
Another told the union: "One child who used hardly any punctuation has been awarded four points while another who correctly punctuated has been given two points."
A poll of 48 heads in one authority showed 47 complaining of "serious inconsistencies" in marking. The discrepancies emerge just two weeks after an independent review recommended scrapping the writing test and replacing it with internal teacher assessment.
A Department for Education spokesman said: "Schools which disagree with the results for any pupils are able to request a review of the marking." He added that markers received thorough training.