SATs tests for 11-year-olds are being sabotaged in an "active campaign" by individuals opposed to Government reforms, it has been suggested.
A Department for Education (DfE) source blamed a "rogue marker" for the leak of an English test due to be taken by youngsters across England.
It is the second time a paper has been published online in recent weeks.
The answers to the Key Stage Two grammar, punctuation and spelling test are understood to have appeared on a website for an English exam board on Monday evening, where they remained in a password-protected area for several hours before being removed.
A DfE source said: "While the test doesn't appear to have leaked into the public domain and can go ahead, a rogue marker did attempt to leak the test's contents.
"It is clear there is now an active campaign by those people opposed to our reforms to undermine these tests and our attempts to raise standards."
The DfE said it was urgently investigating how the tests appeared on the site for exam markers working for a Government contractor.
The leak was not judged to be significant enough to cancel Tuesday's test, with the number of visitors to the site thought to be in the dozens, but Labour said the gaffe was a "body blow" to parent and teacher confidence in the Government.
A DfE spokesman said: "We are aware that Pearson, the external marking supplier responsible for Key Stage Two tests, published the Key Stage Two grammar, punctuation and spelling test on its secure marker site for a short period of time. We are urgently investigating this breach.
"Unlike the Key Stage One test, we have no evidence to suggest this was leaked into the public domain by the time schools began to administer it. The integrity of the test has not been compromised and schools should and must deliver it as planned.
"The site can only be accessed by Pearson's approved markers, all of whom are under secure contract. Any distribution of materials constitutes a clear breach of that contract."
Last month a spelling test due to be taken by thousands of seven-year-olds was scrapped after it was accidentally released online.
The section of the Key Stage One final exam was published as a sample paper on the DfE website on January 26, which a DfE spokesman described as "deeply regrettable".
Shadow education secretary Lucy Powell called for an emergency review of the primary assessment system in light of the most recent breach.
She said: "The possibility that education ministers have compromised the Sats Key Stage Two spelling and grammar test coming, as it does, hot on the heels of their cancellation of the KS1 spelling and grammar test due to incompetence, calls into question the ability of ministers in the department to properly manage our education system.
"This news undermines the validity of the Sats spelling and grammar test children are sitting today and is a body blow to parent and teacher confidence in the primary assessment system."
Christine Blower, general secretary of the National Union of Teachers (NUT) said the latest leak is a "disaster" for children, schools and teachers.
"After months of confusion and mismanagement, they mark the dismal culmination of a dreadful year for primary pupils and their teachers," she said.
"They constitute an experience which must never be repeated; those who have engineered it must be held to account."
Chris Keates, general secretary of the NASUWT teaching union, said: "Given the high stakes nature of the testing for teachers and school leaders, if the integrity of the tests cannot be guaranteed then it is absolutely clear that they cannot be used to judge the performance of schools.
"The time has now come for the Government to commit to conducting an open review of all of the issues surrounding this year's Key Stage One and Two tests and assessment arrangements, including the content."