SATs markers say they are working flat out to hit their deadlines after the Qualifications and Curriculum Authority admitted some were halted by mistake.
Almost 600,000 Year 6 pupils took papers in English, maths and science in May. Their results are due back in schools on July 7.
Fears of delays have arisen after dozens of English paper markers were wrongly stopped following a mistake on the dummy paper used to check their accuracy. If they fail this benchmarking, they are stopped and their papers redistributed.
As the benchmarking process went on, it became clear that some highly rated markers were failing.
A QCA spokesman said: "The problem was that the benchmarking wasn't doing its job ... There were markers who, from their previous record, should be good who weren't getting through. It led to the conclusion that it was the test itself that was the issue."
The difficulty meant alternative evidence on marking quality had to be gathered for those who were stopped. The QCA has now adjusted the difference between the markers' scoring of the dummy scripts and the senior marker.
One marker, who spoke to The TES anonymously, said: "Lots of people are in limbo with partially marked allocation. I had enforced breaks while waiting for benchmarks to reach my team leader and feedback to be given to me.
"I have been told the deadlines are now fluid. I have a week and a half to complete my remaining allocation. It's going to be unpleasant to mark that hard, but Edexcel knows we're going to pull out all the stops for the sake of the children, and is capitalising upon that."
The QCA warned ministers in January that there was higher risk of delays and mistakes this year. This was because of the short timeframe it had to find a new contractor and set up the system after the previous five-year contract with ETS Europe, the subsidiary of a United States firm, was terminated early when last years's results were not delivered on time.
A spokesman for the QCA said: "At this point in time, it's on schedule, but we cannot foresee other issues that may arise during the process. The issues we've had so far have not knocked the timeline off."