Delegates at the National Union of Teachers' conference backed a campaign to abolish the tests which are coming to many schools in September.
Teachers warn the literacy and numeracy tests would stress young pupils.
Schools minister Nick Gibb said it was "extraordinary" that teachers' unions could not say "a single positive thing about England's schools". These "baseline tests" in reading, writing and maths, to be carried out when pupils begin school, are intended to provide a starting point to measure progress against through primary school.
Ministers have argued that the assessments will help to make sure pupils leave primary school having made good progress in these basic skills.
The Labour party also supports the introduction of the baseline tests for reception pupils.
The Liberal Democrats support the tests and reject the way they have been characterised at the teachers' union conference.