The testing system introduced to measure the progress of reception pupils in England is not fit for purpose, according to a new Government report.
Plans to introduce baseline assessments for children in their first year at school were announced in 2014.
The first tests - designed to find a starting point from which to measure progress throughout primary school - were taken in September last year.
Schools were given the option to choose between three different testing systems.
But a study from the Standards and Testing Agency published on Thursday said they cannot be used to compare pupils as the tests from the different providers are not "sufficiently comparable".
A statement from the Department for Education said: "The study concludes that the three different assessments are not sufficiently comparable to create a fair starting point from which to measure pupils' progress.
"As a result, the results cannot be used as the baseline for progress measures, as it would be inappropriate and unfair to schools."
Instead, Key Stage 1 markers will be used to measure this year's reception children's progress.
The National Association of Headteachers said the problem was "symptomatic of the general chaos on assessment in the primary phase".
The Department for Education said it will be considering the options for testing arrangements for those who start reception in 2016 to be announced "in due course".