THE SNP faces claims that it has buried a key report that would shed light on the party's performance on education in a bid to keep it secret from voters until after the Holyrood election.

The results of surveys on literacy and numeracy, which include data on defining election issue of the 'attainment gap' between rich and poor pupils, have been published in March or April every year since the first report was released in 2012.

But this year, the findings will not be published until the last day of next month - three-and-a-half weeks after the country goes to the polls - leading to accusations that the move is an attempt to hide statistics that may point to declining standards and a growing gulf in achievement between children from wealthy and deprived backgrounds.

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The Scottish Government claimed that publication dates of the Scottish Survey on Literacy and Numeracy (SSLN) varied from year to year. It said that as the data would not have been ready until after the pre-election period began on March 23, when restrictions on what information can made available by the civil service kicked in, it had been scheduled for release after May 5. A spokesman added that the publication date had been announced well in advance of the rules coming into force.

However, the Government's own guidance states that regular statistical releases, a category the SSLN would fall into, can continue to be published despite the pre-election rules.

The episode raised further questions over the politicisation of the civil service under the SNP, with opposition parties claiming that it had been complicit in allowing Nicola Sturgeon's party to escape scrutiny over what, along with taxation, has become the major issue of the election.

The row erupted as education dominated yesterday's campaigning, following the First Minister's claims that closing the attainment gap would be her priority mission if re-elected, but opponents accusing her party of not doing enough to tackle the issue over the past nine years.

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Willie Rennie, the Scottish Liberal Democrat leader, said: "It is unacceptable that the publication of these important education statistics has been delayed and just a little bit too convenient that the SNP are escaping scrutiny.

"The results of the last two of these surveys have shown overall literacy and numeracy standards are slipping and the attainment gap is widening. These secret attainment statistics need to be published now so they can be part of the big debate on the future of our education system."

The 2014 literacy survey, published on April 29 last year, reported that there are significant gaps in attainment between children from rich and poor backgrounds. The performance of poorer students in reading had declined compared to two years previously, the first time the literacy survey was carried out.

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The bi-annual 2013 numeracy survey, the results of which were made public in late April the following year, also revealed stark differences in attainment, with the proportion of pupils performing well or very well from the least deprived areas 28 percentage points higher than pupils from the most deprived areas. The gap had widened compared to two years previously.

Campaigning yesterday, Ms Sturgeon said that raising educational attainment should be the "number one priority" of the next Scottish Government and highlighted a string of manifesto commitments aimed at improving education. They include offering more autonomy to headteachers, investing £750m over the next five years in raising attainment and new steps to widen access to universities.

However, she has resisted calls from Labour and the LibDems to increase income tax and spend the proceeds on schools and colleges.

Scottish Labour leader Kezia Dugdale called on her SNP counterpart to commit to protecting education spending in real terms, saying the pledge had been absent from her manifesto.

Responding to Mr Rennie's comments, an SNP spokesman said he had a "brass neck" to talk about education standards as Scotland had "plummeted down international education rankings for maths and reading between 2003 and 2006 when his party and Labour were in charge."

He added: "The situation has improved again under the SNP, with the recent OECD review finding that we are above average in international rankings on science and reading and that there are clear upwards trends in attainment and positive destinations - Mr Rennie should get behind those efforts instead of always talking Scottish education down."