THE Treasury has been accused by Nicola Sturgeon of becoming "transparently party political" after Chief Secretary Danny Alexander used figures compiled by its officials to claim the SNP's anti-austerity plan would raise Britain's debt levels.

The accusation has come in a letter to Sir Jeremy Heywood, head of the UK Civil Service, who last week informed the First Minister of Prime Minister David Cameron's decision to turn down the SNP's request to have talks with civil servants about its election policies ahead of May 7.

Despite the SNP potentially holding the balance of power, it was made clear that only "principal opposition parties", which field candidates across the whole of the UK, could be eligible for consideration to talk to Whitehall mandarins. The Nationalists denounced the decision as "undemocratic".

But following the Treasury attack on the First Minister's anti-austerity demand last month for an extra £180bn of extra public spending in the next parliament, she has fired off an angry letter to Sir Jeremy.

In it, Ms Sturgeon decried Mr Cameron for "falling far short of the standards of inclusiveness, which apply at Holyrood" by blocking the SNP from having access to civil servants.

But she said the position was worse, saying: "It is clearly the case that the UK Government is including the SNP in political attacks prepared at taxpayers' expense while continuing to exclude us from the pre-election access to civil servants.

"This inconsistency reeks of hypocrisy," she declared, "as well as a Treasury which has become transparently party political."

The FM reminded Sir Jeremy about his point that Whitehall would need to be aware of the main policy aims of the minority parties should discussions become relevant following the election.

But she stressed: "However, it would appear the Treasury is maintaining such an awareness in order to mount pre-election partisan attacks on the SNP, presumably at the behest of Ministers, their special advisers and with the full involvement of the Treasury Permanent Security while the Prime Minister rejects the pre-election access I requested."

The Permanent Secretary is Sir Nicholas Macpherson, who courted controversy when during the referendum he published his opposition to independence. In January, he defended his public intervention, suggesting because the "fabric of the state" was at risk of destruction, the normal rules of civil service impartiality did not apply.

In response to Ms Sturgeon, a Treasury spokesman said there was "nothing new" in the UK Government producing costings of political parties' proposals.

"In fact," he explained, "successive administrations have accepted that since departments provide factual answers to MPs and peers about the costs of identifiable changes in activities or benefits, there is no objection to officials providing Ministers with similarly factual information about clearly identified alternative policies."

He added: "Costings are produced by officials in accordance with a clear and established process."