A teacher who discovered that a spelling test due to be taken by thousands of children in England had accidentally been published online months in advance has said the mistake makes a "mockery" of the exam system.
Charlotte Smiles, a teacher in south-east England, said the error was noticed on Wednesday as the school conducted an official trial of the exam when one pupil seemed familiar with the material.
After speaking to a parent at another school she realised the exam had been made available as a sample paper on the Department for Education (DfE) website on January 26.
A DfE spokesman described it as a "serious error" and "deeply regrettable".
Around half a million seven-year-olds will take the Key Stage 1 spelling and grammar tests in May when they are at the end of Year 2 and the results are used to measure the progress of pupils.
Ms Smiles' school had been selected at random to take the paper early to set a benchmark for national standards.
She said the error would give some students "an unfair advantage" and may "inflate" results at schools which had been encouraging parents to use the DfE website to prepare children for the exam.
"They are then going to be higher than the benchmark and it is going to make it look like the national curriculum is really working. And I don't think it is," she added.
Ms Smiles 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 headteachers 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."
She added: "They are expecting us to hold very high standards in terms of administering and keeping these papers stored safely, locked away, so that it's supposed to give a true reflection of the child, which tests very rarely do anyway.
"And it makes a bit of a mockery of the whole system to have accidentally posted it online."
A DfE spokesman said: "This is clearly a serious error and we have launched an immediate investigation to understand how it happened. The material has been removed from our website.
"Fortunately, this is a Key Stage 1 test which is provided to schools to support Teacher Assessment judgements. The data used to judge the performance of schools and the progress children are making at Key Stage 1 are teacher assessment judgements. The results of these tests are not collected.
"Nonetheless it is deeply regrettable that it has happened."
He added: "We ask that if anyone has seen the material, they do not share it further so that the test remains helpful for those teachers who have not yet used it with their pupils."