Nicola Sturgeon has launched an attack on the civil service, accusing it of acting in a "transparently party political” manner.
The Scottish first minister's assault on Whitehall impartiality follows a Treasury analysis of SNP plans to increase departmental spending in the next Parliament.
Treasury Secretary Danny Alexander seized on the policy costing yesterday to say Sturgeon was "factually wrong" in her claim that growth in departmental expenditure of 0.5% a year could reduce debt as a share of GDP in "every year" from 2016-17.
But in a letter sent to cabinet secretary Sir Jeremy Heywood, Ms Sturgeon said it was "clearly the case" that the SNP was being included "in political attacks prepared at the taxpayers' expense".
The first minister also hit out at David Cameron's decision - conveyed in a private letter from Sir Jeremy published in the media last week - not to allow the SNP pre-election contact with the civil service.
"This inconsistency reeks of hypocrisy, as well as a Treasury which has become transparently party political," she said.
Under rules set out in the Cabinet Manual, pre-election contact with the civil service is granted to the "main opposition" parties to allow them to ask questions about departments and inform officials of organisational changes they plan to make.
David Cameron granted Labour such contact in October last year, but the SNP argued that its potential influence in a hung parliament meant it too should be allowed access to officials.
Peter Riddell of the Institute for Government think tank - which has issued a series of reports on pre-election access - has, however, questioned the SNP's call for inclusion.
"The key thing is, are you a credible national player to be in government?" he told the BBC.
"No one denies the SNP could have a massive... electoral and political influence. But they're only putting up candidates in 59 seats. And it's highly unlikely they'll be ministers even thought they could have a big influence on policies of government. So it's being a national force that's the key."