The Government put off the publication of key stage 3 English results this week because the marking of 75,000 papers has not yet been properly checked.
Problems with marking of the tests were revealed by The TES last month.
The Department for Education and Skills announced that it was postponing publication because its head of statistics could not yet say whether the results data, and accompanying commentary, would be "fit for purpose".
The DfES is not giving details about weaknesses in the data. However, The TES understands there have been two problems. Most significantly, inadequate support for markers meant that 100,000 of 600,000 pupils' papers were not double-checked before the results were released to schools last month.
About 25,000 have since been checked, with pupils' levels rising in about a quarter of cases.
With 75,000 papers still to be checked, headteachers' leaders said this meant the overall percentage of pupils achieving level 5 could rise by as much as three per cent.
The Government has not yet said when the results will be released but the Secondary Heads Association said the extra checks could take until October.
Heads' unions also say they have been deluged with complaints from schools about poor marking this year. The problems follow the introduction of new marking processes.
National results for KS2 and KS3maths and science will be published as planned on August 24.