Primary schools across England finally received national test results for 11-year-olds today after a week's delay.
The results had been due for publication on 8 July, but an administrative fiasco delayed marking.
Teachers were able to access Key Stage 2 test results online early this morning.
But around 5 per cent of schools are unlikely to have received their English results.
At an emergency Commons committee meeting yesterday afternoon, Ken Boston, head of the Qualifications and Curriculum Authority, England's exams watchdog, told MPs that while 100 per cent of Key Stage 2 marking was complete, it had not all been entered on the electronic data system used for recording marks.
He said that as of Sunday night, 94.4 per cent of English results were complete and 97.3 per cent of maths and science.
Results of national tests for 14-year-olds (Key Stage 3) are due to be published at the end of the week.
Dr Boston said marking of maths and science was "substantially complete" but that English takes longer to mark and some results may not be available until August.
The children, schools and families select committee also heard that American firm ETS Europe which was hired to carry out this year's marking may face penalties running into "tens of millions of pounds".
Dr Boston said the QCA was "exploring all possible commercial and legal avenues to ensure that suitable action for this failure is taken."
He told the committee: "I think there has been severe reputational damage in relation to the failure at this level, which any large company would be concerned about.
"There are financial penalties that could run into tens of millions of pounds."
Dr Boston apologised for ETS's failure to meet the 8 July deadline and said he was not officially notified that they would miss this until 26 June.
He listed a host of problems including problems with recruiting and retaining markers, and delays in getting papers to them. Markers had also been given wrong information about the time and location of training, call centres were unable to deal with calls and there had been "slow and unpredictable" data feeds. Some unmarked scripts had also been returned to schools.
Despite Dr Boston's assurances that 100% of marking of Key Stage 2 is complete, boxes of unmarked test scripts are still waiting to be collected, it was reported today.
BBC News online reported that English tests taken at several schools were sent to a headteacher marking maths papers at another school.
He has unsuccessfully made repeated efforts to have them collected for marking, it said.
A photograph sent to the BBC website shows six boxes of unmarked English Sats papers at a Lancashire primary school.
It is believed the boxes could contain the work of six or seven primary schools, the BBC said.
One of those affected is believed to be St Martin's Primary School in Runcorn, Cheshire.
Headteacher Phil Hollman discovered his pupils' English results were missing when he checked the marks published online earlier today.
He told the BBC website: "The children have put their heart and soul into this. That's what hurts. I've had to speak to the children this morning to tell them nothing has come back.
"It sums the whole thing up. It's the children who are suffering."Reuse content