As hundreds of thousands of children anticipate their Sats results this week, teaching unions have claimed that up to a fifth of some results could still be inaccurate.

Following last summer's marking fiasco, which saw the results of tests delayed, the Qualifications and Curriculum Agency (QCA) said it was confident that test results would be delivered to schools on Tuesday.

But the National Union of Teachers (NUT) and the National Association of Head Teachers (NAHT), said that based on previous experience, they still had concerns about the quality of marking, and again urged ministers to scrap the tests.

New contractor Edexcel has a target to get 99.7% of all results back to schools on time, the QCA said.

The two unions previously announced plans to ballot over boycotting next year's Sats if they are not abolished.

NUT general secretary Christine Blower said: "What we can say is previous experience suggests that one in five of the results are inaccurate, there is no reason to believe it will be any different this year.

"There is the whole process of appealing which is a hideous waste of time for teachers and upsetting for the children.

"It is time the Government grasped the mettle and accepted there are lots of lines of accountability and that what they need to do is get rid of these tests."

Last year, just over a fifth of English papers taken by 14-year-olds were sent back for review, but the QCA spokesman said reviews for other subjects, in both age groups came nowhere near that figure.

Sats in English, maths and science were taken by 11-year-olds in May. Schools secretary Ed Balls scrapped Sats for 14-year-olds last October, and the science test will be abolished from next year.

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