A spelling test due to be taken by thousands of seven-year-olds next month has been scrapped after it was accidentally released online.
The section of the Key Stage 1 final exam was published as a sample paper on the Department for Education (DfE) website on January 26, which a spokesman described as "deeply regrettable".
Following an urgent investigation into what the Standards and Testing Agency (STA) called a "human error", schools minister Nick Gibb decided to remove the requirement for schools to give pupils the test for this year.
In a statement he said that the move was to "remove any uncertainty and clarify the situation for schools".
He said: "Schools will still need to submit a teacher assessment judgment based on pupils' work in the classroom as has always been the case.
"However there will be no requirement this year for them to administer the KS1 grammar, punctuation and spelling test or use the result as part of that assessment.
"Our immediate inquiry has shown none of the other KS1 test papers have been affected by this error.
"This is a clearly regrettable incident and I am sorry for any concern it has caused teachers, parents or pupils."
He added that he had also commissioned a "root and branch" inquiry into the operations of the STA.
Around half a million Year 2 pupils were set to take the spelling and grammar tests and the results are used to measure the progress of pupils.
The mishap was noticed by a teacher who was trialling the official test when a pupil appeared to know all the answers.
The blunder prompted a fierce backlash against assessment in primary schools, with the Association of Teachers and Lecturers (ATL) labelling it "farcical", while the National Union of Teachers (NUT) called for primary assessment to be cancelled for 2016.
STA chief executive Jennifer Coupland said she believed the issue arose as a result of human error by a member of staff within the agency and a "failure to follow appropriate clearance processes".
Labour schools spokesman Nic Dakin said: "This is complete and utter chaos from Tory ministers who are losing control of their department.
"It is the children, parents and teachers that are waking up this morning to the news that all the work they have put into SATs preparation for the spelling and grammar test is for nothing who are paying the price for the failure and incompetence of ministers."
Christine Blower, general secretary of the NUT, said: "Following the Government's U-turn over the baseline assessment of reception children, this is another blow to an assessment system which is over-centralised, mismanaged and incapable of supporting children's learning.
"Nick Gibb has now been forced to make significant improvised changes at every level of the system - from reception to Key Stage 2 - that he introduced, against the weight of professional opinion, at the start of the school year.
"It is no longer possible to claim that we have a robust assessment system that can track pupil progress from four to 11, nor that schools can be held to account on the basis of reliable data.
"There is every reason to cancel the whole programme of primary assessment for 2016, and to begin again, with genuine consultation on proposals that can command the support of teachers and parents."