Technical errors could prevent parents getting English, maths and science results for children aged 11 and 14 until next autumn.
Despite a long-standing promise, the Qualifications and Curriculum Authority has failed to deliver the computerised results system that was for the first time to list test marks and grades, pupil by pupil.
Instead, schools must trawl through the raw data and convert it to official test grades if they want to publish results before the summer holiday.
The failure will also delay primary schools' attempts to set their own progress targets as the process relies on current results for key stage 2 pupils.
According to the QCA, an outside contractor is to blame for "slippages in the planned schedule". It has set up a hotline for worried schools.
In a letter to all schools with 11 and 14-year-olds, QCA chief executive Nick Tate promises that a new contractor will deliver the results as soon as possible.
"The QCA is naturally very sorry for any inconvenience this causes schools," Dr Tate said. "We are doing all we can to keep the delay to a minimum."
As a result, heads have been told they do not need to report their full results until the next academic year.
The error is said to have alarmed ministers who regard the issue of bureaucracy as politically dangerous.
Teachers' leaders were swift to condemn the additional workload. "This could be the straw that breaks the camel's back," said David Hart, general secretary of the National Association of Headteachers. "Given the concerns about bureaucracy, schools need this like a hole in the head.
"I understand that this may not be QCA's fault. But if we were guilty of this sort of error in the run up to an inspection, we would be subject to severe criticism."
"One of the reasons schools do not do these calculations is because they are are so heavily overworked," said a spokesperson for the National Union of Teachers. "They do not need the additional bureaucracy."