SNP leader Alex Salmond has accused Prime Minister Tony Blair of hiding the truth about the Government's future plans for the Barnett formula.
The SNP says the Government is keeping secret material requested under the Freedom of Information Act on the formula, which governs the proportion of government spending allocated to Scotland.
Scottish nationalists want to obtain documents discussing the future of the formula and its "reform, amendment or scrapping" compiled since 1997, but have been told by Downing Street that it is being kept secret on the grounds of "public interest" and specifically because it relates to the "formulation of government policy."
Mr Salmond has written to the Prime Minister questioning the decision not to release the information. He wrote: "There are two points. First this decision makes a mockery of the Freedom of Information legislation. Second, what do you have to hide?
Speaking at a press conference this morning, SNP depute leader Nicola Sturgeon said: "It may not be in the political interest of the Labour Party for the truth to emerge, but it is certainly in the Scottish public interest to know how the Prime Minister and his advisers have been plotting or preparing to scrap, reform or amend Barnett."
Mr Salmond added: "This is an iceberg that can sink Labour's campaign in Scotland - Blair's Barnett cut will make Tory cuts look like chicken feed."
The SNP is also making the case for the Scottish Parliament to take control of Scotland's oil - particularly in light of high oil prices.
The SNP leader added: "Britain has been taking Scotland's oil for far too long. Now, it is time for a change and SNP success is the key. We can finally claim our big win on the natural lottery, if we make Scotland matter on polling day in May."