Sats tests' results could be delayed after marking was set back by problems in checking examiners.
According to the paper, some of the dummy papers provided by the Qualifications and Curriculum Authority (QCA) to test markers' accuracy were flawed.
To counter the delay qualifications provider Edexcel has taken on additional staff and asked markers to take on more papers.
ETS, the US firm previously tasked with marking the papers failed to get them back on time and lost its contract.
"Marking is well under way and we are working hard to deliver results on time," a spokesperson from the QCA said.
The issue appears to be confined to the quality assurance papers for English markers.
"We are currently working to establish precisely how many English markers are affected. At this stage, we are confident that many will be cleared to continue," the authority added.
While teaching assistants
may not be involved in marking the Sats papers, they do regularly help evaluate the performance of pupils in schools.