The Government is to plough on with single-level tests (SLTs) despite an admission from the agency overseeing the pilot that the trials posed "substantial challenges".
In its first overarching report, the Qualifications and Curriculum Authority (QCA) said that, despite problems raised by the first three sessions, the results provided "firm evidence" for the continuation of SLTs.
The tests are now being piloted by 370 primary and middle schools and are designed to measure whether pupils have reached a particular national curriculum level.
The QCA acknowledged that the tests would continue to throw up "technical issues" while they were in development, but said it was now "well placed" to see how SLTs perform as an accountability measure.
But an unpublished report that emerged last month, written by senior National Assessments Authority officials, warned that the tests faced "substantial and fundamental problems".
The Liberal Democrats said the whole experiment "needed to be scrapped", adding that the QCA report merely showed how difficult the tests would be to roll out.
David Laws, the party's education spokesman, said: "A huge amount of time and money has been invested in these pilots and ministers are now desperate to make them work.
"This evaluation reveals how complex and confusing these tests will be to roll out. Instead, we need more teacher assessment with external checking to ensure that high standards are maintained."
He added: "Ministers have already admitted that earlier reports on the SLTs would not be published. There must be concern that this evaluation does not give the full picture of how flawed these tests really are."