Primary school headteachers have raised concerns over the Conservatives' plans to move Sats tests to the first term of Year 7 because they fear pupils may be deliberately marked down.
The heads are worried that secondary teachers could give pupils poorer marks in order to make it look as if they are making greater improvements with their new pupils.
The Conservatives announced the plans on Sunday, stating that many secondaries already retest their new intake of pupils to assess their literacy and numeracy levels.
The party said the new proposals would remove this duplication, lower costs and improve school accountability.
But speaking to The TES, Andrew Carter, headteacher of South Farnham School in Surrey, said he feared the plans would result in a lowering of standards as secondary teachers vied for better value-added scores.
Mr Carter said: "There is a question that secondary teachers would mark the children down as that would give them better value-added scores.
"What they might find is that the children are actually clever and that they will have to work harder to improve them. So this new system could see a lowering of standards so teachers can show they are improving their pupils."
Robert Trawford, headteacher of Walsall Wood Primary School, West Midlands, went a step further and said he would not want secondary schools validating his school's progress.
"I would be very unhappy if the fate of my school were in another school's hands," he said.
Michael Gove, shadow schools secretary, said he would be open to suggestions about more objective methods of assessment, but added that this was ultimately a question of trust.
"The process of marking would be monitored, moderated and supervised so as to ensure consistency," he said.
"But if people are saying there is this risk, then what they are saying is that they do not trust teachers.
"I would like to see how we could try to bring more trust back into the profession."
Analysis, pages 24-25.