The government has been forced to cancel its controversial new spelling and grammar test after it was accidentally put online by officials.
Schools Minister Nick Gibb said the incident was "clearly regrettable".
Half a million seven-year-olds in England had been due to take the tests next month, as part of their Sats.
Head teachers' leaders had called for the test to be scrapped as it had been used as practice by an unidentifiable number of schools.
On Thursday, the BBC News website reported that a spelling test from this new additional paper had been available on a Department for Education website, for use as practice material, since January.
The blunder was initially spotted by a teacher at one of 700 schools that was carrying out an official trial of the test, using the paper that was to be taken by pupils around England.
Russell Hobby, the leader of the National Association of Head Teachers, said: "We have no way of knowing how extensively it has been used by schools and parents."
Mr Hobby said the schools minister had acted "quickly and appropriately" in cancelling this part of the Sats tests for seven-year-olds.
Mr Gibb issued a statement saying: "To remove any uncertainty and clarify the situation for schools, I have decided that we will remove the requirement on them to administer the Key Stage 1 grammar, punctuation and spelling test for this year only."
He said that no other test papers for Key Stage 1 pupils appeared to have been affected.
The schools minister also announced there would be a "root and branch inquiry" into the Standards and Testing Agency, an agency of the DfE that sets tests.
Jennifer Coupland, chief executive of the Standards and Testing Agency, said the blunder had been caused by "human error".
She said they were alerted that a sample paper posted online contained the live test words rather than the sample test words and removed the document from the website as soon as they were made aware.
The test, known as the SPAG, was being introduced this year for children in Year 2. Teaching unions criticised the move saying it introduced a technical approach to language for children too early.
Hundreds of thousands of papers, amid high security, will have been sent to schools in readiness for the statutory test to be taken in the first week of May.
Teacher Charlotte Smiles, who spotted the error when one of her pupils seemed to know all the questions before they were asked, said the mistake showed a "lack of management" and had made a "mockery" of the system.
"We received our papers last week and they are currently locked up in a secure location where only one person is allowed the key - and the spare key has got to be locked away in the safe as per all the guidance," she said.
"We are subject to quite a few security rules and if we don't follow those... we can have maladministration proceedings brought against us.
"That can spark an Ofsted [visit], and teachers and head teachers can lose their jobs.
"Yet the DfE have published it online anyway and I just wonder are they going to be suffering the same proceedings that we would."
Labour's shadow schools minister Nic Dakin said ministers were losing control of their department.
"Children, parents and teachers, 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, are paying the price for the failure and incompetence of ministers."
Responding later, a Department for Education spokeswoman agreed the incident was "regrettable" but said government reforms would improve children's learning.
"We are unapologetic about making sure every child is getting an education that helps them to reach their full potential," said the spokeswoman.
A guide to Sats
Key Stage 1 Sats are taken by six- and seven-year-olds in England at the end of Year 2.
Pupils take tests in:
- English grammar, punctuation and spelling (two papers)
- English reading (two papers)
- Mathematics (two papers)
The Key Stage 1 test results are used by teachers to reach an overall judgement of the standards pupils have reached in these key subjects.
Parents also receive a teacher assessment for science, though there is no science test.
A second set of tests are taken by 10- and 11-year-olds at the end of Year 6. These Key Stage 2 tests are used as a measure of school performance.