Examiners work the weekend to clear key stage 2 and 3 test backlog and get results to schools
The accuracy of marking on millions of key stage 2 and 3 papers is in doubt and examiners fear many will not be returned to schools by next week.
All test papers are due back to primary and secondary schools by Tuesday. But in at least three subjects - KS2 English and KS3 reading and maths - marking will continue over the weekend, with some markers warning it may take longer to clear the backlog.
But any delay could have serious repercussions for schools, as parents are supposed to be told their children's results by the end of term.
The marking of the 9.5 million papers, sat by 1.2 million pupils, is being overseen by ETS Europe, a US-owned company that insists its system of computer-based checks will make the process more reliable.
But senior examiners have warned that quality control is weaker this year, so pupils may receive less accurate results.
For the first time, rather than having a sample of their scripts checked, markers have shown their competence by assessing a hypothetical script online.
A senior examiner said at least one marker had been given papers despite many errors, while others feared that markers could put extra effort into the hypothetical script then rush through the real papers.
Also for the first time, papers have not been remarked this year if they are just below the threshold of a national curriculum level.
A senior marker said: "There are always one or two pupils where you make clerical errors, which are corrected second time round. That won't happen this year."
In previous years, these "borderline" checks have marginally increased the national test results. Government statisticians have warned that national pass rates will drop by up to two percentage points as a result of the change.
Examiners have been working seven-day weeks in emergency centres set up in hotels in Leeds and Manchester to finish the marking.
ETS Europe has been paying all their expenses, including hotel bills, meals at pound;20 a day and taxi fares.
Despite these last-ditch efforts, one KS3 reading marker working at a marking centre near Manchester airport said: "I don't think they are going to hit the deadline."
ETS staff were also this week trying to persuade markers to take on KS2 English scripts at home, even though the deadline for home marking passed last week.
ETS Europe has faced huge criticism after a series of problems with the administration of this year's marking. More than 90 examiners have contacted The TES to complain and thousands of criticisms have been posted on our website.
Mick Brookes, general secretary of the National Association of Head Teachers, said he was extremely concerned. "It just underlines our view that the assessment system has to change," he said.
Despite the backlog, some schools received their papers this week.
A National Assessment Agency spokesman said: "Over 90 per cent of papers have now been marked and the results entered on the system. The NAA expects marking quality to be similar to previous years."
11th-hour marking, pages 16-17.