It has been suggested that SATs tests are being sabotaged in an 'active campaign' against Government reforms after an English test due to be taken today was accidentally published online.

A Department for Education (DfE) source blamed a 'rogue marker' for the leak of the test.

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.