The Scottish nationalists have accused Labour of hatching secret plans to cut spending north of the border.
The spending allocation for Scotland is made by the Treasury each year
The claim came after Downing Street refused to release documents on the future of the Barnett formula which determines Scotland's budget.
But Labour said it had nothing to hide and the formula had been successfully used for many years.
The Liberal Democrats and the Conservatives said the government should not hide any plans for change.
The Scottish National Party revealed that under the Freedom of Information Act it had asked the government to reveal what the future held for the formula.
The request was rejected because the government claimed it related to the "formation of policy" which was exempt from the act.
SNP leader Alex Salmond said: "It seems it isn't just the Tories who have a secret agenda of spending cuts for Scotland.
"The prime minister's office has now confirmed that Labour are hiding a potential billion pound attack on Scottish public spending - even admitting that the secret material is about 'policy formulation'.
"This is an iceberg that can sink Labour's campaign in Scotland - Blair's Barnett cut will make Tory cuts look like chicken feed."
But Scottish Office Minister Anne McGuire said the government had nothing to hide.
She added: "The SNP have been trying to undermine the success of the Barnett Formula for years.
"They know that their economics are superficial and shallow and the Barnett Formula has worked."
The formula was created in the 1970s to allocate public spending for all of the nations and regions of the UK.
Labour said the formula had always produced a favourable spending outcome for Scotland.
However, the Liberal Democrats Jim Wallace said that both the government and political parties should "square with the people about their intentions for the future of the Barnett Formula".
He added: "We have already published some proposals and we have debated them at party conference as to how we might take forward the way in which we should fund the Scottish Parliament."
The Scottish Tories' Peter Duncan said that the government should reveal all it knew and should not hide from tackling the issue.
Council tax cut
The Tories were also keen to put council tax on the Westminster election agenda, claiming the tax could fall by 35% under a Conservative administration.
Mr Duncan claimed that council tax in Scotland had gone up by more than 55% since 1997 with "no noticeable improvement" in services.
He said: "All over Scotland, people are thinking what we're thinking - council tax is too high and too much money is being wasted."
Meanwhile, the Scottish Green Party said it had serious hopes of gaining its first Westminster seat.
Robin Harper, the party's co-convener, said: "Our biggest attraction for the voter will be when they know that the Greens are the only party to connect social justice, the environment and the economy."