It is thought English papers will be affected most
The pass rate in England's national Sats tests could fall by up to 2% this year after a change to the marking system, it has emerged.
A practice known as "borderlining", where papers were re-marked if they fell just below the expected level, has been dropped this year.
The exams regulators had recommended the change, saying there were other ways of assuring quality of marking.
The Conservatives say the system was exaggerating England's test results.
Exam papers which were marked as being just over the pass level were not re-marked.
A study by the National Assessment Agency (NAA) - which now regulates the tests, taken by 11 and 14-year-olds - concluded that the change could lead to a drop in the numbers meeting the levels expected by the government.
A statement on the website of the Department for Children, Schools and Families (DCSF) says: "Analysis of results data for previous years by NAA allows us to estimate the impact of the process changes.
"This analysis indicates that the removal of borderlining in 2008 is expected to cause a fall in the proportion of pupils achieving the expected level by up to two percentage points."
It predicts the impact of the change will vary between Key Stage (age group), level and subject and that English will be affected more than mathematics or science.
Shadow children's secretary Michael Gove said: "The credibility of the exam system has been put in danger by this whole process.
"Ministers cannot escape their responsibility for tests which parents and teachers say they can no longer trust."
The change was proposed by the exam regulator in December 2004.
Results from Key Stage 2 Sats tests (taken by 11-year-olds) have improved over the past 10 years.
Last year's results show that 79% achieved the level expected for their age in English, 76% in maths and 87% in science.