The exam board responsible for running this year's tests for 11-year- olds will have to use markers with no primary experience to fulfil its contract, experts predict.
Edexcel is contractually obliged to recruit 30 per cent more markers than it needs for this summer's KS2 tests, as the Government and exam regulators seek to avoid a repeat of last year. But the Chartered Institute of Educational Assessors believes staff who marked the defunct tests for 14-year-olds will have to be used.
David Wright, the institute's chief executive, said the 130 per cent proviso was "overly cautious".
The 130 per cent rule was revealed in a letter from Christopher Trinick, chairman of the Qualifications and Curriculum Authority, to Ed Balls. He says some extra markers will be "held in reserve".
"All markers will be able to elect to take on realistic additional allocations of scripts," he wrote to the Schools Secretary. "Centralised marker panels will be held in readiness as an additional contingency."
This week, Edexcel said it had already recruited 3,570 of the 5,400 markers needed for this summer's tests. It has until the end of April.
A spokeswoman said so far all the markers had previous experience of marking KS2 tests, but QCA guidelines would allow it to use experienced KS3 markers if necessary.
"Basically our priority is getting the KS2 markers, and if there is any problem with that then we will go for the other markers," she said.
A QCA spokesman said there had never been major use of secondary teachers to mark primary tests because of the KS3 Sats.
"All markers go through a rigorous training programme specific to the tests they will be marking and are closely monitored to ensure they mark to the high standards required," he said.
Last summer, the test results of 1.2 million pupils were delayed. ETS, the American contractor, bore most of the blame. Disorganised training, computer glitches and missing scripts did not help. Lord Sutherland's inquiry revealed long-standing fears about whether enough markers has been recruited.
Three years ago, Edexcel advertised for graduates without teaching experience to mark GCSE and A-levels, for a fraction of the rate.
Mr Trinick's letter also reveals concerns about the practicalities of delivering scripts to markers.
"A major obstacle is that many of the markers are also teachers who are unable to receive home deliveries during the school day," it warns.
Extended delivery times and allowing deliveries to schools might help, the letter added.
QCA and government officials are considering this month whether on-screen marking could be introduced from 2010.